In Mathematica 10, it seems that the Alt+= and Alt+- shortcuts affect the entire cell instead of only the selected text. In the following example, I highlighted "line 2" and pressed Alt+= several times. The result is shown for Mathematica 9 on the left and Mathematica 10 on the right.
I also ran into a strange situation in Mathematica 10 where the keyboard shortcuts changed the size of the entire cell except the selected text, but I wasn't able to reproduce this behavior. When I change text size from the Format menu, everything works as expected (i.e., only the selected text is affected). It's just the keyboard shortcuts that are acting strangely. Is there any way to fix this?