I have seen some mats cut with a wider bottom. Why is this done?
The practice of bottom weighting mats is a subject of great debate in the framing world, and I have heard a number of explanations for its function. One suggestion is that framed pictures used to be hung higher on walls, and that weighting the bottom helped to compensate for the angle of viewing (this never made much sense to me- in that case, you would need to weight the TOP to overcome visual foreshortening). Another common explanation is that many works of art are created with the focus of the image slightly below center, and that weighting the mat helps to keep the image from looking like it is "sinking" in the frame.
The explanation that makes the most sense to me is that many galleries (especially in photography) will reuse the same frames from exhibition to exhibition, and when the frame is a different shape than the artwork, you need to absorb the difference in the matting. Often, the extra border width is simply placed at the bottom of the mat (perhaps because this way it looks intentional, rather than a side effect of recycling frames?) People have grown to associate this "look" with gallery framing, and sometimes emulate the design even when it is possible to order a frame the same shape as the artwork.
With the exception of "gallery" framing, most framed artwork I see nowadays is framed with even matting, so weighting the matting is far from a requirement. If you choose to create this look, I would suggest that you make the variance in border width large enough so that it doesn't simply look like a mistake (that is, make it noticeable) but not so large that this design element distracts from the image. I'm thinking something along the lines of 20%-30% (for example, four inches top and sides, with five inches on the bottom).
