_transient* options in wp_options
I’d be happy to look into this issue for you.
Transients are meant to be, well, transient. They are supposed to clean up after themselves after a set amount of time. That being said, you can always delete transients without fear of losing data or breaking anything.
Looking into it, I see that this has been reported by other users, and there is a ticket created for it. I will try to dig up some more information to see why it’s happening and what can be done about it.
In the meantime, you can use a plugin like Transient Cleaner to remove them.
I will report back here when I have some more information.