You did a "clean" install. As far as Time Machine knows, you've essentially got a new machine. So it's looking for a place to put that new machine's data.
Time Machine backs up want you want, not what you don't want. It will eventually delete what you don't want as it needs room for what you do want. It sounds like you want archiving, and it's a different "backup" strategy. It's keeping a fixed copy somewhere, not an iterative backup of your current computer, which is what Time Machine does. The strategies overlap; I have a Time Machine backup of my current iMac that holds family photos, but I also keep an archive of the originals offsite.
If there are things you don't want to keep on iMac because you want it "lean" (whatever that means), then find some space outside of Time Machine to store it so that you'll ALWAYS have it. And you could include that archive in a Time Machine backup as well for redundancy.
And Time Machine is probably gonna tell you it's starting a new backup. Remember, you did a "clean" install, so you got what you wanted, a chance to start everything over. You can still restore/migrate stuff back to your iMac from that Time Machine backup, but it's probably not as efficient as archiving copies of what you want to keep and discarding the rest.
Rob