This may be an issue, I converted my gallery from PP to vba and had been doing thumbs at 100x100 with PP. I decided to leave vba at 150 which appears the default and thought I would rebuild all of the 5,000 late at night after a while. I discovered that it only rebuilds the size of the number you select to process at a time. In other words it does all 5,000 images but if I set the number to process at a time to 10 it only does the first ten to 150x150 and the rest remain at the 100x100. If I select the first 100 then it will do the first 100 and the rest will remain the same size. It does process all of them because it change the thumbs, I can tell this because images that were watermarked now have watermarks on the thumbs and did not before.
I was lucky that only about 90 images had been uploaded before I discovered this so I have resized those to 100x100 now and my gallery looks normal but thought you might want to know this.