For pngs (and lossless encoding) the thing is that the last step of encoding is a run of the deflate algorithm. It's impossible to say what exactly a stream wil

For pngs (and lossless encoding) the thing is that the last step of encoding is a run of the deflate algorithm. It's impossible to say what exactly a stream will deflate down to without actually running your implementation of deflate. Possible encodings of an image pre-deflation are staggering, 5^height just for the filter step on all color types. I've been writing a brute force compressor (everything but the deflate step which depends on the implementation you choose) as a joke and it takes about an hour on images 10px high. For that reason encoders use heuristics to make best guesses whenever there's a choice to be made.