Brain size is not necessarily a good proxy for intelligence.
Humans (with some notable exceptions) are the 'smartest species' on earth, but we don't have the largest brains, whales are definitely larger and horses are a similar size. The ratio of bodymass-brain can be used as a proxy; but it isn't sufficient to explain the apparent difference in intelligence between species, with ants, birds and mice scoring better than humans.
Perhaps of particular interest is that the brain structure of warm/cold blooded animals is quite different, with reptiles being more intelligent per brain mass than might be expected. I struggled to find good information on insect brain structures, but an alternative structure could provide more oomph at a small size.
EDIT: Having done some further reading on insect brain structures, they are vastly different from mammals. They are much more tightly integrated to the sensory inputs (eyes, olfactory etc.) than mammalian brains. You would likely either want additional insectoid brain features not seen in earth or a mammal-like brain/body with insect-like body parts.