Let's be blunt — there's no room inside a tank for tall, broad-boned people.
There are exactly three constraints:
1. Hatch diameter. That one's self-explanatory.
2. The crew's working space. To keep a tank from being an easy target for artillery, tanks are made as low as possible. The most popular tank in the Soviet army, the T-80, stands 219 centimeters tall. Everyone's seen a G-Wagen? A G-Wagen is only 20 cm shorter. Now picture: the hull, packed with the engine, transmission, and other technical guts. On top of the hull, a turret with a gun and other militaristic guts. Got it? Now picture packing all of that in and still leaving room for the crew.
3. If you've pictured how point 2 is supposed to work, here's a new variable — armor thickness. The thicker the armor, the longer the crew's life expectancy. And per point 2: to keep the tank from being an easy artillery target, that extra armor thickness gets added on the inside.
It's unclear from the text exactly which tank the narrator encountered. If we assume it was an M1 Abrams, the United States' main battle tank during the Iraq War, its height is 237 centimeters. Now look at the picture. There's the loader, walking around inside the tank at full height.