You're using the term "Beam" and speaking about jon boats (typically flat bottomed boats). Flat bottomed boats width is measured at the base of the transom, so a 1442 will be 42" wide along the base of the floor. The beam measurement is the widest point from the top of one gunwale to the other perpendicular to the keel line - for instance a 1436 flat bottom boat has a 57" beam - but would be considered to be 36" wide.
Utility boats are measured at the beam and are sometimes referred to as jons but are measured according to their beam width. So you can find a 1457 utility offered by Lowe right now - but it isn't 57" wide at the base of the transom.
If you're talking about flat bottom boats the difference in stability will have a lot to do with the length of the boat. The difference in stability between a 1442 and a 1448 won't be nearly as extreme as it would be between a 1242 and a 1248. Also, sometimes as you get into wider boats you start getting into designs with higher sides, so you can easily get into an apples an oranges comparison if you're not looking at all the dimensions all the time.
It isn't quite as bad as women's clothing sizes, but its far more convoluted than it ought to be.