It's a rather old question, but let me clarify a bit:
Rikki-Tikki-Tavi got confused by logic gates versus logic cells - A typical logic cell of a typical FPGA is much more complex than a single logic gate. Some rather rough conversion factor is about 100. Hence, these FPGA are much much more powerfull than needed for this rather purpose.
A FPGA that most likely could do the job (despite the space rating and temperature range) can today be bought for few (single digit) Dollars measuring 4x4 mm^2 - and that's just limited by the amount of inputs and outputs and not by the size of the silicon itself.
Sure, radiation hardness, temperature rating and the ten year old hardware used on the rover pays its toll with respect to size (not mentioning the price rising by many orders of magnitude), but you'll get the point.