A run like 0000000 is only compressible if it appears more often than it would in a fully random output. And in a fully random output, we would expect the run '

A run like 0000000 is only compressible if it appears more often than it would in a fully random output. And in a fully random output, we would expect the run '0000000' to appear about once every 16 bytes. When you compress 0000000 to something smaller, you have to replace '0000000' in the plaintext with a smaller symbol in the squash text. Which means if that smaller symbol appears in the plaintext, you have to replace it with a larger symbol in the squash text. And if your smaller symbol appears more frequently than '0000000', you are going to end up with a "squash" text that is larger than the plaintext.