Xterm loses part of its screen content when it changes from a small to a bigger font: the lines get clipped to the visible area occupied by the previous window size in the smaller font.  This shouldn't happen... should it? The screen and scrollback buffer should be totally separate from the font size used. 
The clipped screen content is irreversibly lost,  and copy and paste will actually reproduce the clipped lines from then on.

Speculation as to what might cause this:
Could it be that xterm confuses resize and font change events in some unfortunate way?  E.g. when an xterm changes to a smaller font size, it calculates its new size, resizes the window, but catches the resize and calculates new buffer dimensions based on the old font size, thereby clipping the lines and losing the screen content?


Reproduction instructions:

Open an xterm, switch to a big font e.g. "Huge", create some screen output, e.g. with "ls -l", then switch to a small font e.g. "Tiny", and back to "Huge".