This happens because the average velocity of gas molecules depends on the temperature.
The equation that gives the velocity of gas molecules explains quite well how the temeparture and velocity of molecules relate such that:
`v = sqrt((2kT)/m)`
k expresses the Boltzman's constant
T represents the temperature
m expresses the molecular mass
Notice that the velocity of gas molecules is proportional to temperaure, hence, the velocity increases if the temperature raises.
Hence, since the smell of food is a gas, the hotter the food is, the faster the smell reaches you and the colder the food is, the slower the velocity of aroma molecules is.
The molecules expand and spread far apart when heated, which is diffusion of gases in air due to the constant motion of gas particles