Dovetail jigs n00b question

I'm looking at dovetail jigs on marketplace because I'm interested in doing some joinery. Now my question is I have a palm router that takes a 1/4 and I see some of these jigs say they have a 1/2 template which i don't understand since the bit carves out the dovetail so why does the bit size matter?

What am I missing here? Also are there dovetail jigs compatible with a 1/4 bit?