W.I.T.C.H. had a couple of jokes relating to the girls receiving a certain kind of "enhancement" during their transformations; From the one-hour series premiere "It Begins/It Resumes":
Hay Lin:(looking at her new wings) What do we have behind us? Cornelia:(looking straight down) What do we have in front of us?
There's an episode with the word "wenchy" in it, with the word "outfits" after it.
There's an episode where the word poo is used, but afterwords when it seems like someone is about to say it, a horse neigh cuts the sound off, indicating she was going to say shit instead of poo.
And a later joke in the second season episode "W is for Witch", when the girls were able to appear in their transformed states out in the open because they were able to pass it off as Halloween costumes. The girls all look visibly buxom, but Cornelia's mother is unimpressed by their "growth"...
Mrs. Hale: Didn't think those costumes came with that kind of padding...
The comic was much more explicit about it—one instance involved already-busty Irma using her "growth" to enhance her chest size to attract a boy, which doesn't work out the way she wished, and she ends up embarassing herself. And an episode of the both the comic and TV series involved them being stuck in Guardian form, with Cornelia (the one above who was pleased with what was "in front" of her) pondering if she could get older boys now in the TV show and Irma wondering the same in the comic.
Similarly, in the comic, resident Pettanko Will (short for Wilhelmina) is quite insecure about her underdeveloped bust size and actually prefers being in her Guardian form because it makes her breasts bigger. Most images of her in Guardian form involve her arching her back and sticking out her chest.
Later Cornelia gets a temporary upgrade of power which makes her substantially more busty, and while Caleb, her boyfriend, is "hearts-for-eyes" pleased, Irma crosses her arms across her chest and looks very embarrassed while saying something snippy.
The novels based on the comics are even more explicit about it. Taranee, the young black girl with Hartman Hips, is shown here as a Pettanko who is initially jealous of Irma. Conversely, Irma is described as an "early bloomer" and much hay is made about how buxom she is, with both Will and Taranee showing open jealousy and bitterness.
Irma focuses on her breast size as much as Will but for different reasons. Irma is bigger than the other girls, in human form and Guardian form, and is rather prideful about it.
Also from the premiere, something that made this troper actually rewind to make sure I heard it right: When Hay Lin first starts to get her powers, she begins sneezing and wrecks her room(her sneezing powering the wind), gets somewhat worried, and goes to talk to her grandmother about what happened. Her grandmother is trying to keep the Heart of Candracar hidden and isn't paying full attention to Hay Lin, who gets this annoyed look on her face and asks "This isn't another one of those 'becoming a woman' things, is it?"
Irma's line about "that really wakes up your underwear" when she falls out of the portal in "It Resumes". I'm not sure what that means but I'm pretty sure it's this trope.
The advertisements of the comic when it was first published described Irma as "also generous in her shape". "Generous in her shape" is Italian slang for "big tits, big butt, thin waist". It was targeted to teenaged girls.
For some reason, Will's Guardian form is often depicted sticking out her breast.
"E is for Enemy" has a scene where Irma and Cornelia are sleeping together, in a closet. Cornelia is draped across Irma, resting her head on one breast while her hand is on the other, while Irma's hand seems to be stroking Cornelia's hair... and they're both very comfortable-looking as they sleep curled up together. Best not to look too deeply into this...
Probably the most surprising and explicit was Nerissa explaining how she masqueraded as Julian's lover in order for him to impregnate her so she could have a child; which pretty much amounts to rape.
"Ghosts of Elyon" has a scene where Will goes to sleep in Elyon's abandoned house. On top of the covers is her jacket and pants. While she is probably still wearing the shirt that was under the jacket, the fact that we don't see anything but her head and her hands until she bursts out from under the covers fully transformed suggests otherwise.
For a cartoon that on the surface is so bright and cheerful, W.I.T.C.H. also has a great deal of genuine horror, but one key example that truly pushed the content envelope was a scene in Z is for Zenith In that episode, Cedric betrays Phobos, transforming into his serpent form and eating him, and then going on a rampage in Heatherfield.
In "A Service to the Community", the image of an unconscious Will with white paint on her face is rather ... questionable.
In "Stop the Presses", Hay Lin gets knocked onto the back of baby pig, and Newton's 3rd Law ensues when the pig stops running. This gives the viewer a very nice double panty-shot. First when when Hay Lin is in the air (including sprawled arms and orgasm expression), and second, when she lands bottom-up against a wall. The worst part is that in the mid-air panty-shot, a close inspection reveals a cameltoe. Seen from about 2:09-2:14 http://www.youtube.com/watch?v=PHkNDShXUug
At one point in the comics, they managed to show Phobos getting changed after one of his baths - with some whisperers holding up a cloth as the sole form of censoring his privates.
In K is for Knowledge, Teranee is unavailable, so Matt offers to fill in for her. Will replies she'd rather not see him "filling in her thights."
In the last episode of Season 2 (animated series), during the final battle, Cedric swallows Phobos. Irma and Hay Lin comment on the fact, then Matt says 'So don't want to be around when he-' before he's cut off by someone.
In the penultimate episode of Season 2, the guardians remark on how they've been fighting Phobos constantly since the events in the previous episode. Hay Lin calls these events 'Phobos's Halloween Coming-Out party'.