I'm using gpg to encrypt and sign a file on a linux machine.

using the same keys the file is encrypted and signed on a windows machine and the file sizes are different.

I then encrypted and signed on another windows machine and again the file sizes are different.

Does GPG use some random stuff from the machine during encryption?

Using the same keys, or the same keyphrases? When you generate your keys, GPG uses "some random stuff from the machine", but after that those keys are not changed; in fact if they were you would not be able to decrypt your files.

If you use the same keyphrase on another machine or the same one to generate a new key, you will have a totally different key. Perhaps that is the answer to your question?

Try decrypting the file from one machine with the key from another machine. You should generate your keys once, then export and import thme to other machines as needed. Make a backup copy of your keys and store it in a safe place!