As others have mentioned, foot fetishes are extremely common, and there’s a lot of overlap between feet and size content - on multiple fronts. I’m not a foot person myself, but this is what I’ve observed:
Fetish-wise, there’s the obvious dom-sub dynamic, themes of humiliation, dehumansation, etc. I would also add to this: reverence. Feet have historically, and for many cultures, been associated with reverance and a show of respect (this is still hierarchical in nature so is not mutually exclusive to dom-sub dynamics), e.g. kissing or bowing at the feet of someone, worshiping the ground someone stands on, washing someone’s feet (this one can alternatively be also a show of care).
Physiologically, feet are common erogenous zones. People like having their feet touched because it feels good (or they like touching other people’s feet because they think it will feel good to the other). But also some people just simply find them appealing in some way; aesthetically, out of fascination, whatever the reason may be.
Narratively, it can also make sense to have feet as a significant or primary theme to a size story. Like, if tinies and giants stand on the same ground, giants’ feet would be one for the first things for tinies to notice. It’s also a very perilous position, which is always great for action/drama.
But, to talk to your specific point about seeing a lot of feet in size content lately: honestly, there’s always been a lot of this - see above - but it probably seems more obvious due to greater use of generative AI making image creation much easier and accessible, and able to manifest more people’s ideas/fantasies. Not to make this a discussion about AI (that really isn’t my intention), that’s just what I end up seeing if I go a-browsing for more size content.