I suspect you're using black backed mounts where the mount covers up the stamp box printed on the page. If so, having the very visible mount extend beyond the box where the stamp gets mounted does look a little odd sometimes. That's because with black mounts your eyes sees the black "frame" around the stamp, one of the reasons many collectors like black mounts, in fact. You can't not see it.
However, rather than searching for the perfect sized black mount which will fit neatly into every stamp box on every page (which would then be quickly discontinued if you had my luck!), what might work much better is to use clear mounts. With clear mounts you don't really notice the mount very much at all. You see the stamp and its frame printed on the page. A slightly larger mount that goes beyond the frame hardly matters. Might be a good reason to switch. This is the reason, I suspect, that all major album manufacturers use clear mounts on their hingeless pages. It just looks better, it's cleaner, and they do not cover the stamp boxes.
Pages with mixed black backed and clear mounts, by the way, have never bothered me -- though some people don't like it. Those people are wrong, of course.

I mix stamps in mounts with hinged stamps, so why should I care if I also mix some black mounts with clear mounts? They all look good to me.