Starter motors take lots of current - up to and in excess of 100 Amps - smaller cars a bit less.
The junction at the Positive lead very probably has a high resistance. Just a few ohms and a high current will generate lots of heat.
Inspect the crocodile connector for a poor connection - peel back any sleeve and have a good look - crimp in a vice or solder if you have a big soldering iron.
Heat = Watts
Watts = Amps squared x Resistance
Just half an ohm at 60Amps will make 1800 Watts
This is why...
Peccavi
December 2012