The reason: The larger the lithium cell is the more heat it absorbs. The ratio of heat-radiating surface to volume is less favourable than for flat 20-ampere-hour cells. Large cells like that are often in a plastic case with walls one centimetre thick.
How to get the heat away?
Not because the cells are particularly sturdy that way, but to protect the cells from diffusing water vapour. Plastic is much less expensive than a metal housing, but it needs to be thicker to act as a water vapour barrier. The thick wall, in turn, impedes heat dissipation, as does the material. (HS)
Look at this, too:
Storage advice: Lithium storage units can actually burn
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