The secrets can be found in certain puzzle rooms as broken walls that lead to a room containing the secret.
As for the size of the game itself I think you are getting confused. The Linux version was added as experimental because I could not test it as I didn't, and still don't, a system with Linux. You can delete those files in order to save space.
As for the rest of the size, I'm sorry but the audio was not compressed and sadly my expertise didn't allow me to combine files from one version to the other in V1.2 as it was my first game and I don't think is an easy thing to do. Apologies if you don't like it but it how it is.
