Thank you fellows, Archie and Neal
I checked my /boot folder before running the command Archie suggested, and I had 3 old kernels in there that I don't use anymore and each one has it's own initrd file. Their sizes were ridiculous!! 100MB, 260MB! No wonder the last plymouth theme install I did took 11min! ( I timed it
Anyway, I cleaned out the old kernels, and ran the command and now the initrd file is 3.5MB.
BUT, after ONE
install of a new plymouth theme (only took about 10sec), it's up to 5MB. Definitely something to keep an eye on.