Once you reach a certain (very wide) rim width relative to the tire, this is true, but for the rim widths you would be looking at for a 50mm tire, I don’t think it will work out that way. Might even be taller, but either way the difference is negligible.

There are good reasons to go with a rim wider than 17mm for a 50mm tire, but making the tire less tall is not one of them..

Here is an interesting thread from another forum with a diagram to illustrate: http://forums.mtbr.com/wheels-tires/...th-756818.html