I don't quite know how this works, so forgive me if this is a stupid question, but couldn't someone just take the difference between your inbound and outbound tor traffic to find how much traffic originates from your computer?

If the in/out rate of your bridge was both constant And lower than the max in/out rate of your connection, but it seems a bit of a stretch.

(And of course they wouldn't know that it was your traffic to whatever site they're surveilling, they'd just have evidence that was not inconsistent with you actively using Tor to do Something Or Other at that time.)

They might know how much, but they wouldn't know which traffic was yours.

The same is still true even if you do not run a tor node.

They can see all of the packets in both directions. Which mean they could tell when more was coming out than being relayed in.

Yes, because nothing will make you less interesting to law enforcement than running a Tor node.