Do you like mobility? Being able to easily get in and out of a kneeling/sitting/squatting position without your belt end up at your nipples? Minimal interference with hips/legs so you can run at top speed? Minimal weight in your loadout? Get a thinner belt.
You can still carry plenty of kit on even a 2" belt like the FRV, let alone a 3", and remain comfortable. The Tyr Tac belt that (far as I know) started the movement that FRV copies, comes with a padded mesh inner belt with the velcro interface - as an example. Crye trousers have padding in the waist and I'm sure lots of other clones do too. Not that you always need padding it depends on how your belt gear attaches for the most part, but options are there if you do get uncomfortable. Granted I'd tend to go for something a bit bigger than the FRV style if the belt is carrying all of my gear at that time vs running a belt + PC or chest rig, but you're still better off keeping it narrow and sleek as you can. Not to mention the fact that high mounted pouches/holsters on a tall belt cause all your mags to tip and lean outwards and flap about the place.