I think I figured it out. The 1/2" stuff is 1/2" ID, where the 15mm is 15mm OD. But a lot of suppliers (like Defender) list the 15mm as being ID and since I already knew that the 1/2" tubing was too big it didn't make sense to me that the 15mm tubing would fit.
11-3 - just an update that Whale 15mm tubing was the right size and it seem to be listed wringly as 15mm ID in a lot of catalogs