Are the 5700 and 5800 jockey wheels the same? I know one is 10 speed and one 11 speed but are the jockey wheels the same size so I can put some 5700 in my 5800 mech?

Thanks.

Edit – have some rd5700 but they look a lot fatter hence checking before I pull the mech apart!

I don’t have experience of this swap, but I use Ultegra jockey wheels in a 105 mech. The 105 pulley-bolt is shorter than the Ultegra part and locks the the jockey wheels solid if you tighten it.

The 5700/5800 jockey wheels have different part numbers:
5800 part numbers
5700 part numbers

From your description of “fatter-looking”, I would expect a similar problem. I used 6Nm threadlock on a loose bolt which worked for me, but if it doesn’t work you’re in for a walk home, or worse. Buy matching pulley bolts or the correct wheels