http://upload.wikimedia.org/wikipedia/commons/thumb/0/0c/Namer_in_US.jpg/800px-Namer_in_US.jpg Namer in the US. Note how tiny the soldiers are compared to the vehicle. Average height for US male citizens is 1.77m.
http://en.wikipedia.org/wiki/File:Merkava4-atzmaut1.jpg Note the person on the left next to the Merkava. He can't even look on top of the main hull without tiptoeing. The total height of the tank should be 2.66 meters.
http://upload.wikimedia.org/wikipedia/commons/7/7a/T72_Georgia.jpg Georgian T-72. Height of the hull of this tank is slightly lower than the ingame Slammer.
http://www.supervideo.com/MerkavaMk4Ma.jpg Even granting that tankers are usually not as tall as average (about 1.65 ish in the german army afaik), note how the turret alone is almost as tall as the soldier standing in front of it.
http://i1071.photobucket.com/albums/u510/captain_cave_maniii1/Merkava%20Project%20File/Merkava%20Mk4%20w%20Mine%20Roller/Merkava-Mk4-w-MineRoller0.jpg Another example. Tankers are barely as tall as the hull alone. Following picture of Arma 3 Merkava and Panther to illustrate:
[URL=http://imgur.com/7zfT2T0][IMG]http://i.imgur.com/7zfT2T0.jpg[/IMG][/URL]
Proportions are correct, both vehicles are much too small however.