[simiones]

You then need extra memory to store the de-compressed instructions, since you can't predict before running the decompression how many instructions you'll get. And you still the problem of an unpredictably-sized instruction cache.

Plus, the larger the instruction cache is, the worse every branch mis-prediction is. As far as I know, the size of the instruction cache is not really limited because of space issues, it's limited for precisely this reason.