Three short lines, and a small, awkward moment: someone is getting dressed and finds that their shorts don't fit the way they used to. The speaker says it plainly. The shorts are tighter, and the gut is sticking out. There's no dressing it up and no long explanation.
What makes the poem work is how little it says. The line breaks make you pause on "tighter" and then on "gut," as if the speaker is looking down and taking stock. The word "gut" is blunt and a bit unkind, so it sounds like someone being honest with themselves, maybe a little embarrassed, maybe a little rueful. We can't tell whether the speaker is upset or just shrugging, and that uncertainty is part of the mood.
The tone feels dry, wry and slightly deflated, like a quick private joke at one's own expense. It's a tiny observation about getting older, gaining weight, or simply being caught by a mirror, and most readers will recognize it.