Does AIXI admit the possibility of random string generators? IIRC it only allows deterministic programs, so if it sees patterns a simple model can't match, then it's forced to update the model with "but there are exceptions: bit N is 1, and bit N+1 is 1, and bit N+2 is 0... etc" to account for the error. In other words, the size of the "simple model" then grows to be the size of the deterministic part plus the size of the error correction part. And in that case, even a megabyte of additional complexity in a model would stop effectively ruling out that complex model just as soon as more than a couple megabytes of simple-model-incompatible data had been seen.