Hi antp, I've noticed that compared with a few years ago, the HD images now seem to be less compressed than they once were? It used to be that they would be reduced in size to no more than about 90KB, but now I'm noticing file sizes of 120-130KB are common. Have you made any changes to the level of compression in the last year or so?

I didn't change settings for that since years (86% in progressive mode).
Note that the final size of the picture depends of the details of the picture. A sharp picture with a lot of details will take more space.

But it is also possible that the version of PHP that the site currently uses makes slightly bigger images than the previous one: that was changed around June 2014 when the site moved to another server.

I've checked the file sizes of HD images I uploaded to your site back in 2013 and they are all 80-90KB regardless of image detail. Some of the ones I just uploaded, on the other hand, are up to 143KB in size - or about 60% larger than the older ones. There's a noticeable increase in picture quality/sharpness. It's just unfortunate that this was only changed last year, since the old ones I did are much lesser quality than they should be.

Were you uploading JPEG or PNG back then?
Maybe it is you who compressed them more back then? (as the site compresses pics only if too big)

I've always uploaded in PNG with only a few exceptions. I also don't compress the images on my end...