It's not a very bright idea to park your Humongous Mecha standing straight up. It's a pain to climb back up to the cockpit, it can get knocked over easily (also one of the reasons they don't exist in the real world), plus it just looks a bit silly.
So, make it kneel or curl up into a ball. With the center of gravity lowered, the mecha is harder to knock over. With the height lowered, the mecha is easier to store and can't get in trouble for impeding overhead air traffic. The pilot has a shorter and (usually) easier climb back up to the cockpit. Unfortunately, parking the mecha like that makes it a larger target for dive-bombing birds.
Compare Transforming Mecha, for cases where the mecha's transformation allows for easier storage (and disguise).
Anime & Manga
- In Full Metal Panic!, most Arm Slaves (or at least Mithril's) are stored kneeling down when aboard the TDD-1, or lying prone when transported by helicopter or by road.
- Inverted in Neon Genesis Evangelion, where standing up is the idle position for an Evangelion. It's justified in that it's stored in a huge cage, and the entry plug is inserted into the back via cranes.
- In expanded Gundam works, because they generally operate in zero-gravity, the Zeon space colonists do not store their mobile suits standing upright on racks like their Earth-based Federation rivals and instead have them lying down, back-to-back, or strewn about all over the hangar because there is less of a concept of "up" without gravity.
- Patlabor has Section 2's Ingrams stored standing up when on standby at base, but when it comes time to take the field they are transported lying prone in dedicated trailers that then raise them into a standing position for deployment, a depiction that remains fairly constant across all versions.
- In the first movie the JSDF's Helldivers are also stored lying down in specialised aircraft for airborne deployment. Logical since they're from the same production series as the police's Ingrams.
- FranXX, the titular mecha in DARLING in the FRANXX, are normally stored standing, but since the hangers are within the giant defensive walls of a city, this is a sensible form factor. For transport, however, they're curled into the foetal position to fit inside a cargo plane.
- The Big O: While the titular megadeus normally resides in the protagonist's converted bank mansion it travels the undercity in a humongous Drill Tank.
- Armored Troopers or ATs from Armored Trooper VOTOMS are relatively short for mechs, only standing around four meters tall, but they're often stowed in a kneeling or crouched position. A pilot can board with a pull-up, or perform maintenance on a stepladder.
- Star Wars: B1 battle droids in the prequel trilogy are designed to be able to fold into a compact shape, with the legs folded and pulled up to the chest. This allows for convenient transport and storage; Multi-Troop Transports are able to carry one hundred and twelve of them at once.
- The Boxguards of Deus Ex: Human Revolution are called such because they fold up into a cube when inactive. One memorable room has hundreds of the things on the walls, stacked together in compact form.
- Vital Suits in Lost Planet have a specific "folded-up" mode, so the pilot can climb in and out easily.
- The powered armour units in FEAR 2: Project Origin crouch and open their cockpit when they're usable so the pilot can climb in, or stand up straight with a shield activated when they're in an automatic repair mode (which tends to be how you find unoccupied ones to hijack).
- Starcraft II: Thors have a unique animation that plays when they get picked up by a dropship. They fold up into a more compact block hanging under the ship.
- Plenty of mechs in Mass Effect series, such as geth armatures, LOKIs or YMIRs, assume a compact shape when not activated, making for easier transport.
- One of the activation animations of the MECs in XCOM 2 has them rising from a ball and powering up. Can look silly depending on what the MEC did right before the animation.
- Custom Robo are kept in a cube form, which is launched from a cannon and unfolds at the start of a battle. How the cube lands can impact the fight, since a robo who unfolds right-side-up can move immediately, but one that lands head face down will have to flail about a bit to get on its feet.
- In the Subnautica series, exiting your P.R.A.W.N. Suit while on land will cause it to automatically crouch down, so that you can reach its top-mounted access points.
- The Big Guy of Big Guy and Rusty the Boy Robot sits in a giant chair when he powers down.