Comparing NiMH and LiFe battery capacities for transmitters

[Monkeysmate]

I'm struggling to get my head around the different battery capacities.
My lad's MT-S transmitter takes four AA batteries, and we're using NiMH rechargables with 2900mAh capacity.
I'm contemplaiting getting a LiFe battery for it though, as they seem to last much longer in my other son's MT-4S.

What I don't get is why a 6.6v 1000mAh LiFe battery lasts so much longer than 4.8v 2900mAh NiMH.

Can any of you electrical boffins explain?

Is it just that the NiMH self-discharges at a much higher rate than LiFe?

Ta