A Walking Tank is practical but only in the battlefields we see.As pointed out in the main page, a Walking Tank is Awesome, but Impractical. However, the world of the Dust games is one where the Second World War has continued through 1947 and with more devastating weapons. It's not clear that this is a Crapsack World, but perhaps the battlefields represented in the game are just too filled with rubble and wreckage for anything but infantry and walking vehicles to move through.
The miniatures are bigger than other 28mm scale figures because they are supersoldiers.A fan theory is that the soldiers in Dust are actually bigger than other 28mm soldier figures because they are all mildly enhanced supersoldiers. Not up to Captain America levels, but more than average grunts.