If I move files to a new disk will Time Machine do a full backup? I have a MacMini running 10.12.3 with an internal SSD hosting 2 large iPhoto libraries. I'm going to move 1 of the libraries to a new external SSD (attached to the same MacMini).

What will Time Machine make of this?

I guess it will treat the moved library on the new external SSD as new and make a full backup? And the backup size for the original internal SSD will shrink given that 1 of the libraries is no longer there?

Reason for asking is I'm wondering whether the first TM run after this library shuffle will be much larger and so will delete a lot of old backups to make space (I've not got much spare space on the backup disk).

(It would be nice if TM recognised that it's the same bunch of files - just in different places. But that's probably asking too much!)

Thanks for any support.
Timemachine identifies backups by the disk's UUID. Therefore, the answer is yes. TM will make a new backup of your files and delete old backups if there is not enough space to hold both copies. 

You could delete the previous backup of your iPhoto library and then simply do a new backup. However, you will lose your backup history.

I did something similar a few years ago and I remember that I was able to point TM to the new directory and, therefore, avoid a redundant backup of these files. Unfortunately, I am not entirely sure how I achieved that. This article should get you started with the tmutil utility, which may have the capability to do that.

A final note as you seem to care about your data. You should not rely on one single Timemachine backup. Especially, if you are going to experiment with sudo and tmutil. After some bad experiences, I maintain two rotating TimeMachine backup disk plus a irregular full copy on a third disk.