Why is that. If I went wider then I would be stretching the tire to fit the wheel. I do not have the "bulge" on mine. Never cared for the look, either. Main reason y I went 255 instead of 245 because it looked "stretched". Or I could be crazy or possibly tire discrepancies from the manufacturer.
8" is the minimum width rim that most tire mfrs will recommend you put that size tire on. Some of the difference in appearances between a 255 and a 245 on the same width wheel will come from the two sizes having different "measuring rim widths". Some can come from tires being made by different mfrs or different model tires made by the same mfr - tire dimensions can vary over a small range (at leaat 3% IIRC, which is ±7 mm on tires in the 245 - 255 region).
"Stretched" is a term that doesn't seem to have a clear or widely agreed upon definition. In the strictest sense, you could probably argue that any rim that's wider than the tire's "measuring width" wheel represents a "stretch" and that anything narrower is "squeezed" or "stuffed".
But words like "stretch" imply being outside the tire mfr's rim width range spec for the tire in question, so I think they need to be defined that way.
That way, as long as you're within the mfr's width range, neither term really applies in common conversation. Outside it, additional description such as "mild" for the first half inch outside the range would be at least reasonable (and maybe "crazy" for some of the examples you can find if you Google on the term "stretched tires"). IOW, a 255/45 would be a mild stretch on 10" wheels, a crazy stretch on 11.5's or 12's, and a mild squeeze (or whatever) on 7.5". I don't know exactly how you'd describe the difference between being mounted on 9.5" vs 8", because they're about the same as far as the mfr is concerned.
The first thing you should consider when matching tire size to wheel width is where it falls with respect to the tire mfr's width recommendations. These are pretty consistent across tire mfrs because they ultimately trace back to the Tire & Rim Association.
Anyway, a tire starts out being designed for a particular rim size, and will reflect its "actual" dimensions when mounted on a rim of that width. There is also a tolerance either side from there, generally 1/2" to 1". What that means is that for (say) a 255/45 tire whose "measuring width" is 8.5", it is mfr-acceptable to mount that tire on rims anywhere from 8" at the narrow end out to 9.5". Hence my comment about 8" being a relatively narrow wheel for a 255/45 tire - it's the minimum recommended.
Something about "measuring widths" - for tires above 50 profile, the measuring rim is only about 70% of the tire section width as measured on that rim. IOW, a 255/60 would be measured on a 7" wide rim because of its "tall" profile. 45 profile and lower tires are measured on a relatively wider wheel - I'm pretty sure it's 85%. So a 255/45 ends up being measured on an 8.5" wheel.
What that means in terms of appearance is that a 45 profile tire is specifically NOT supposed to look like a 60 profile tire in terms of "belly" (to use this thread's term). A 255/60 is 10" wide, and will have 0.5 x (10" - 7") = 1.5" of "belly" on both the inside and outside by intentional design. A 255/45, on the other hand, is only intended to have 0.5 x (10" - 8.5") = 0.75" of "belly". 0.75" is just barely enough to put the sidewalls out past the faces of the wheel flanges, and it will look pretty flat-sidewalled by comparison to your memories of the older and taller profile tires - but keep in mind that THIS IS THE INTENDED SHAPE FOR A 45-PROFILE TIRE. They aren't supposed to have bulging sidewalls, and you're not doing the tires any favors by squeezing them down onto narrow rims so that they do.
This is all a consequence of tire design requirements, which with respect to rim width involves things like where the sidewalls flex the greatest and making those regions sufficiently durable in terms of resistance to the heat generated while driving. Outward tire shape appearance hasn't entered the picture at this point.
FWIW, a 245/45-17 is measured on an 8" wide wheel and is typically associated with a rim width range from 7.5" to 9".
FWIW #2, I have put together (and occasionally update as new tire sizes get included) a fairly extensive spreadsheet that deals with tire sizes and rim widths - and the dimensional effects on a given tire size as the rim width is varied. The section width and amount of "belly" (and "apparent profile" and couple other things) vary slightly with rim width.
FWIW #3, mounting a tire closer to the max recommended width can save your a$ $, if you are unfortunate enough to rapidly lose pressure in one rear tire. The additional lateral stiffness that remains in the other rear tire because of the wider rim will keep the rear end under stronger directional control than if the tires were on narrower rims. Guess how I might know this.