"but the last few hours the backups have been about ~70GB each time. What's up with that?"
Suggestion:
If you want clean backups that are "normally sized" and truly reflect what is on your internal drive, stop using Time Machine and switch to CarbonCopyCloner instead.
CCC backups are fully-bootable exact copies of the source drive. They don't "grow" in size any more than your source.
You can set up CCC to clone the recovery partition as well, and CCC can now "archive" older versions of changed files (similar to what T.M. claims to do).
The major difference with CCC is that there isn't a stupid "on/off" switch. You must set it up and run it "wilfully".
T.M. is "backup for dummies". Just turn it on, and you're all set, right?
The only problem is that if you read this forum, you'll regularly see posts from folks to the effect: "I can't access my T.M. backup…" etc., etc.
A CCC backup is instantly mountable, instantly bootable, with all your files right there in front of you in POFF (plain old finder format).
(just waiting for the replies that say, "T.M. is intended for a different purpose… blah, blah…")
The purpose of backing up is to have a backup at a moment when you desperately need it.
When that "moment of extreme need" comes, you'll be better served by having a CCC backup close-at-hand, than with a T.M. backup…