The default “view” in both Nautilus and Thunar file managers is the “Icon“ or thumbnail view, which you can of course change if ever you want via the View menu. If you’re happy with thumbnails, but they’re too big for your liking, or you actually want to make them larger, like in your ~/Pictures folder, hold the Ctrl button while you zoom in or out with your mouse scroll wheel.
In Nautilus (the default file manager in Ubuntu), you can make the thumbnails of pics rather huge, like only 2 fitting in each row, and the icons for any folders in there won’t be too large. On top of that, the change will only be recorded in the folder you’re in, so you can have most of your folders displaying thumbnails at the default size, have a few where the thumbnails are quite small, and then have huge thumbnails for folders like ~/Pictures.
In Thunar (Xubuntu‘s default file manager), you can only go so big, and in your ~/Pictures folder the icons for any folders in there might actually look bigger than the thumbnails for pics; also, do that to one folder, and it is a global change (ie: it happens to the rest).