It sounds like the DPI of your screen is too large, meaning everything will appear larger than it should on a desktop (something could be mistaking it for as a tablet screen with a high resolution)
Sadly, I can't find any "screen scale", "density", "scale" or DPI setting in Ubuntu MATE (in regular Ubuntu, this would be under the Monitor Properties), but this could be a pointer of the issue.
The screenshots on this other thread should also visualize if it's a scale issue: