I think some messages will cost more storage than others. I have 4 devices synced to my Signal account. Yesterday, my friend sent me a 6.8 MB cat video[1]. I presume Signal has to store this cat video until I boot up my 4th device and load the queued messages.
[1]: It was a copy of this cat video: https://www.youtube.com/shorts/8Ud1Cr76j8s