Yes, there is a geeky version, some people like to analyze, visualize or just prefer to be geeked. So here you go.
First a legend, blue is Ergo Sport, brown/pony print is Beco 4th generation, gray is Boba Classic. Oh Snap was so much wider and taller I opted not to include it (plus it is on loan right this second).
I took 1 picture to compare width
That is about how they feel worn. The Beco the widest, followed by the Boba and last the Ergo. You can see how deep the Ergo is in this picture but not really how deep the Boba is. The Boba has some depth but so little that it was hard to catch in a picture. It was not obvious like the Ergo was. The deeper the seat the less close to you the baby is held. With a small baby with short legs and not as much behind to them this doesn't effect as much. But with a taller, bigger child it does. A non darted seat spreads their legs apart as they are hugged close to your body. A darted seat sits them back into the carrier, creating a seat for them to sit on so their legs end up more forward. Their abdomen isn't up against you as much then and their legs aren't as "spread eagle". This makes a big difference in how the width of the carrier is actually used.
The Beco feels wider when worn b/c the seat is all against you. The Ergo feels the least wide because some of the seat width goes out from you not against you. The Ergo still utilizes that width for the comfort of the child though so the legs are still supported. The Boba ends up in between. This really means that it depends on what the wearer likes the feel of. Maybe I should have done pictures of the child seated in it and how their legs are positioned and where the fabric hits, if anyone wants that I can do it when my model is home from school.
I did measure though.
Ergo Sport (the same size as the Original) is 14" wide at all points by 14" tall
Boba Classic ranges from 12.5" wide to 17" wide at it's widest point by 16" tall
Beco 4th generation ranges from 12.5" wide to 16" wide at it's widest point by 17" tall
Some people don't like a darted seat, some people don't like a non darted seat. This very well could be that they are used to the other and therefore are not getting a good seat.
A non darted seat does not create a seat, you have to create it. To do this the waist band should be snug, completely tightened as you will be wearing it before you put baby in. Then the body will go under and then over baby creating that seat. If you do not tighten it first then baby will end up using part of the waistband as a seat.
A darted seat already creates a seat so you need to get baby fully into it. I have heard of people fastening the waist last in order to do this. I cannot judge, well enough, where they should be to do this so still do not get a good seat. Instead I fasten the waist belt loosely. If the waist is too tight you will not be able to get baby back into the seat and all the way down to the waistband because it will be a snug fit. So fasten it loosely so you have room. How loosely depends on baby's size and your comfort level because safety comes first, we don't want baby to fall. My kids are 7 months and a toddler, I've also been doing this for over 10 years, so I can easily fit a fist or 2 in my loose waistband. You lower baby down to the waistband then pull the body away from you as you go under them then make sure to push the seat up snug onto baby's bottom as you lift the body up behind them.
Then for both versions you fasten the chest strap first then tighten the shoulder straps only to take in the slack. Then you tighten the chest strap. Only after the chest strap is tightened and adjusted to the position you want, do you do the final shoulder adjustments. With a darted seat you have to be careful not to overtighten the chest straps as you want baby in the seat, not pulling them out of it. If you start overtightening, the shoulder straps will end up snug under your armpits and uncomfortable. Also, if you start with the shoulder straps too tightened before you put baby on, for front carries, you will have trouble reaching to buckle the chest strap (or getting it over your head if you buckle then put it on as is shown in the Ergo video) and that usually ends up in people wearing the chest strap too high.