Well large, scale size locomotives can't physically make it around sharp curves. In real life or model form. My MTH Premier Santa Fe Texas is not going to go around an 031 curve. The wheel base is too long to even fit. It's rated at 072 but will go around 054 curves without issue.
My layout isn't massive (12x16 or 16x17, depending on how you measure it) but I have 072 curves on one line and 054 on the other. I could have had a more involved track plan with smaller curves, but then I couldn't run scale equipment. So I compromised to run scale equipment. You're not going to be able to run a scale Big Boy on a 4x8 layout. It's just physically impossible to have something that large go around the tight curves required.
As for what's driving large curves it's people wanting scale equipment, which require large diameter curves to operate.
