Both sizes have the same 'section width', that is the maximum width (more or less, rounded to nearest 5mm). The 75% profile tires are a bit 'taller'. That is, the distance from rim to tread is 75% of 225mm v. 70%.
When talk about the 225/75/16 not being as wide, it is usually in comparison to 235/70/16 tires. The 75s are both a bit narrower in section width, but also narrower in tread. Specifically the 75s taper a bit more.
Some (not all) tire 'spec' tables list a tread width.
E owners have been concerned with tread width because one potential rubbing point is the curved top of the strut tower. A square shoulder tire is more likely to rub in this area, and a wider tread tire more so than a narrower one. When you are trying to maximize tire size, those details matter. Knobby tires (e.g. all-terrains like BFG TA KOs) are more likely to have square shoulders. Some even have 'tread blocks' on the sidewall, to protect against rocks, and possibly add traction in deep mud.