Actually, the tissue does not grow -- it is the print shrinking as it dries in the mount press -- usually the print will shrink significantly in only one direction. To prevent this, I put the print in the mount press (without the tissue!) Actually due to the high humidity here, I first dry the mat board (after it heats up in the press, I open and close the press several times to drive out the moisture). Then I put the print on the mat board (without tissue) and repeat the above.
I only tack two adjacent corners of the tissue to the mat board.
PS -- I prefer to mount square images on vertically orientated mat board.
Pendantics - if dry mounting tissue's cut to the print size it bleeds out.