I am by no means an art expert (my only art are just doodles on a sketchpad) so I could be telling you complete crap.
Tanks are HUGE. You already noted it in your title, but your chubby tank looks squished. As for your UN tank it looks a bit short as if a middle chunk is missing. Try to bring some sense of the scale and size of the tank into your drawing.
I'd advise drawing with a picture of an actual tank and try to familiarize yourself with its dimensions and exterior details to make it more realistic. That and maybe add some perspective, unless side views are what you want.
To further
pimp out bling out tacticool your tanks, add more crap to the outside. This is war not some showroom floor at a museum. Add huge slabs of ERA/applique armor, searchlights, grenade launchers, smoke launchers, machine guns, remote weapon stations, periscopes, radio antennas, dozer blades, rucksacks, shelves, grates, jerry cans, tacticool military symbols and unit designation numbers, more turrets on top of other turrets,battle damage etc.
In all seriousness, don't add so much detail that it obscures the tank; add just enough so that the tank looks like a tank ready for battle. Oh and FYI most tracked armored vehicles have more than four road wheels on each side.
