Where are the smallest, lightest hydrocarbons captured in a distillation tower?

At the top of tower.

Explanation:

In a distillation tower of crude oil refining,  the assembly and pressure and temperature conditions are such that we obtain the lighter and low boiling point component at the upper side of tower while the heavier and high boiling point component at the lower side of tower.

From top to bottom the order is

a) butane and other lower hydrocarbon gas.

b) gasoline

c) naphtha

d) kerosene

e) diesel etc

f) heavy gas oil

g) paraffins