Again, "big muscles" is a very big part of strength. Professional bodybuilders
are strong, folks. Those big muscles are not
entirely for show.
And the strongest guys in the world (and gals!) do have "big muscles" as well, even if they aren't shredded for show. Of course, body weight adds to force. So, if you want to be able to move heavy weights, being a heavy weight yourself (

) makes it that much easier.
Finally, Strength is not just about your ability to exert raw physical force, but your athletic training and bodily power.
My breakdown would be the following. Of course, individual cases will vary...
- Bodybuilders have good athletic training, very good bodily power and raw physical force.
- Strongpeople have incredible raw physical force, good athletic training, and good bodily power.
- Gymnasts have incredible athletic training and bodily power, and in many cases good raw physical force.
Of course, when it comes to 5E carry capacity and lift/drag/push, D&D uses the entire score, which doesn't make as much sense for someone like a gymnast, who would have a good Strength score, compared to a strongperson would could lift much more giving the same Strength score. There is also the impact on Athletics, breaking/bending, and attack bonus/damage.
- In terms of the Athletics-aspect, gymnast would benefit most by far, strongpeople next, and bodybuilders last.
- In terms of breaking, bending, etc. the strongperson would benefit most, then the bodybuilder, and last the gymnast.
- In terms of attacks and "hard hitting" damage, it isn't quite as clear-cut IMO. Obviously, being bigger and heavier, strongpeople (and to a lesser degree bodybuilders) would benefit, but gymnasts also have a great amount of explosive power in their movement, so I don't think would be very far behind, but probably a decent amount back.
A note when it comes to the attack bonus from Strength: IMO that comes from the hard-hitting aspect, being able to penetrate armor (natural or otherwise) and partly from bodily "control" (an aspect of bodily power maybe?). Now, there is some ambiguity when it comes to the last part, since traditionally most people contribute that to Dexterity (agility and balance, both associated with hand-eye coordination).
That's how I balance it all in my head, anyway.
