The bottom line is that they were all different serums with different administration techniques. Even Irskine's super soldier serum, had it been administered to a second person without the vita radiation used as a stabilizing agent, might have had different results than the administration of the serum to Steve Rogers. And remember that Irskine's serum, at least, only unlocks the maximum human potential; it doesn't push anyone past it. It just so happens Steve was practically a cripple (at least on paper), so the change is far more pronounced in him.
That aside, Isaiah Bradley is buff and tall. Remember that he is at least 91 years old, and despite that, he still looks like he could probably give Luke Cage a run for his money. I wouldn't want to mess with this guy in real life, let alone in a world where he's super powered:
Bradley was pulled from Army duty in 1951 to be experimented on to take on Bucky as the Winter Soldier during The Korean War, and he shows up in The Falcon and the Winter Solder in 2024. Since he would've had to have been at least 18 (or 17, possibly, if he lied about his age) to enlist, that would make him at least 90, probably 91 or 92, depending on his birthday.
Remember also that Steve looks pretty frail when he's older, too, even with the serum:
Granted, Steve is technically 181 years old here (or arguably 115, if you don't count his 66 years in the ice), so about twice as old as Bradley.
Regarding the winter soldiers, from what I recall they all looked pretty buff in the movie, too, so what makes you think they didn't also get some kind of buff boost? I also recall the serum they received was more potent than the one that Dr. Zola used on Bucky Barnes, because they were able to overpower him fairly easily in terms of pure strength. From a strategic perspective, being able to provide the same or greater strength without making someone look like Conan the Barbarian is a big asset, because someone can blend in with crowds of "average" people more easily. As you can see from Captain America - The Winter Soldier when Cap and Black Widow try to go incognito at an Apple store, they clearly stand out; they're literally remarkably fit (as shown by the employee referring to them as "[ideal human] specimens" (OK, well, it is an Apple store, but still...).
Given that in The Falcon & the Winter Soldier, Dr. Nagel (the former HYDRA scientist recruited by the CIA to recreate/emulate Irskine's original formula using Bradley's blood from vials in storage) claimed he was able to recreate it perfectly, except by "improving it to not alter the person's physical appearance", this shows that such a variation is indeed seen as an advantage, or improvement, over Irskine's formula. It also explains why those who took Nagel's serum, like the Flagsmashers or John Walker, didn't have their physical appearances altered at all.