Presumably you equally expect halflings to be built like brick sh*thouses and so on? Gnomes to rippling little bodybuilder dudes?
Halfling and gnome Strength scores are ridiculously high given their size. That's why 4E upped halfling height to four-plus feet. And if you look at the 4E halfling art, most of them
are pretty ripped. There was quite a bit of conversation around this when 4E was coming out, with the art team pointing out just how small "3 to 3.5 feet" really is. We're talking preschoolers and kindergarteners here.
I have other reasons for disliking halflings and gnomes, so I don't fuss too much about their Strength scores. If I can swallow their presence to begin with, I can choke down their Strength as well--in the end, it's just another modifier. But any time I stop to think about it, it bugs me, and has ever since 2E.
Otherwise that's some strange-as-heck logic. One could equally assume that their muscles were different - a chimpanzee and a human of equal strength typically have wildly different-looking levels of bulk. Hell, humans of equal measurable strength can have quite different degrees of visible bulk.
Chimps hide their muscles under a coat of hair, but they have a
lot of muscle. Google for "hairless chimpanzee" and look at the image results. (Fair warning if you do, though: muscle isn't the only area where some chimps have a lot going on.) "Brick sh**house" is an accurate description.
Now, I grant that bulk isn't everything. You can be strong without being a hulk, and in a fantasy world, you can always fall back on "A wizard did it." But, once again, we are talking about creatures the size of 5-year-olds. There are limits to what wiry strength can do.