I use SketchUp too, and I think I know what's happening there... it's caught me, as well.
When you stretch or shrink text, (or any object, really...) there are THREE handles at each point - Top, Bottom, and Middle.
If you pick the top or bottom handle, then the text THICKNESS will be scaled diagonally through the center along with it's height and breadth! That's probably what's happening, especially if you're grabbing a corner.
If you want to adjust the size in only ONE dimension, then you have to scroll in VERY close so that you can see the MIDDLE handle. When you select that one, you will adjust ONLY the object's X, Y, or Z dimension without affecting the others. This will let you make those fixes without changing the thickness.