Let's take two chips; one with leaky transistors and one with not so leaky transistors.

the chip with not so leaky transistors consumes 125W under load. That 125w is converted to heat.
the chip with leaky transistors consumes the 125W plus more (let's say 140W) because some power is being lost due to the leaking. that 140W is being converted to heat.

The leaky chip is outputting 140W of heat compared to the 125W of the not so leaky chip. It runs hotter.