If it is over 75 degrees, you should get an air conditioner, to keep it cool. If it is too hot and they can die of heat stroke. Sometimes genetics has to do with size. There could be many reasons why he isn't as big as the younger one. It could be that he hasn't had another growth spurt yet, it could be that the genes he was passed on, his body structure will be different than the younger one. Maybe he will be leaner and longer bodied than the younger one? Just like people, chinchillas do come in different sizes and shapes.
It could be that he might be a runt in a litter or that he is a dwarf. There is lots of reasons why an older chin may end up smaller than a younger one.
With your two, it could also be that if one is a mutation, some mutations seem to grow slower than say, standards. Standards tend to be full grown in a year, some mutations tend to take up to two years to fully grow.