I agree.. we had the monterey for my son, who is built like a linebacker (aka, like his dad)... and has super broad shoulders... it was a 10 for him... with the old belt guides. I wish they hadn't tried to make it fit every kid... but they wanted that best bet rating on the IIHS testing... I really think it was better before that. The probooster is awesome for slimmer kids (Jeffrey would have just outgrown it in height when we got it for his sister, but he couldn't sit back into it because the wings don't expand and his shoulders don't fit), which makes it a 10 for HER... now she has such a long torso that her knees only JUST folded at the edge when we bought it for her... so the Vivo before that was her perfect booster as it has such a shallow seat... it isn't as tippy, but the probooster does not fit well in our third row height-wise, anyway, and I don't need it to like I did need the vivo to sometimes... Damian and Ruthie get along better now.
Ruthie is so narrow still in the shoulders that she doesn't get a good fit in a parkway, which has a narrow bottom but wide set shoulder guides... her brother would have done well in it except he might have run out of room for his booty.
Jeffrey was able to get a good fit in any backless booster as long as he was the right height for head support.