Photoshop: Similar raw files compressed as JPEG files are different sizes

It's possible there's a lot of edit metadata in one of the files and not another. Are you saving using Save As or Export > Save for Web? If you export using Save for Web (and whack the metadata in both exported files) are they they same size?

thanks for the quick reply. Yes, when I save for web, they have about the same size. But the metadata is minimal in both

I wonder whether this is really due to the compression in jpg formats, which occurs regardless of the quality setting. If you have a large area with similar colors in it, this compresses to small file size because the jpeg compression does look for differences from the average within each block being encoded.  Note that the previous comment applies even with highest quality. The difference really depends on what you mean when you say that the files are "similar." In particular, are they pictures of the same thing? If that's the case, my comment is probably not relevant. But if the subject matter is different - big blue sky in only one, for example - my comment does apply.

they have the same subject, I bracketed the shots. The first larger one is slightly darker, the second has a bit more whites in it, but it seems impossible that this could account for a 50% reduction in file size.

It's possible that the darker image has more graininess or is somehow more uneven to the compression algorithm, causing it to be less efficient at blocking similar colors together at highest quality compress, while smooshing them together at lower quality compression.