Breaker tripping wires disconnected
The conduit size will not affect the current, you probably have skinned the insulation and that will cause tripping like you have if the breaker is a GFCI. If a non-GFCI the skinned part is much worse but could still be the cause.

I would get a megger (high voltage ohm meter). I have seen hand crank meggers for around $100 this would probably be cheaper than having an electrician check it.  You disconnect the hot from the breaker and put 1 megger lead on the hot 1 on the ground and crank, with the switch turned off or disconnected the reading should be high 100 mega ohms. If it is low - 10k or less - you have damaged insulation. 

I have seen this many times as you found pulling the #12 UF in conduit is a real chore. I am surprised you got it in 1/2", but both metallic and pvc conduit can easily skin the wire and cause the problem you are having.

Most folks don't realize conduit needs to be chamfered to reduce insulation damage when pulling.