Your average fatass gym lifter should have no problem finding clothes, as much of today's clothing is designed for fatasses.
For those who lift and have some decent muscle but maintain low bodyfat and a small waist, though, there is a problem: Shirts that will fit your chest and shoulders always have a shitload of extra material around the waist, as they are designed for fatasses.
Likewise, pants/jeans with the correct waist size never fit your thighs (with the exception of "ultra-baggy" style jeans).
The thing about cars/ airline seats/ etc is total bullshit, though.