The way I think of it is that the heil can only carry as many people as the rows on the sides, unless the dropship is wide enough to have at least a 1 tile wide column between the side and center seats, as you aren't going to stand without any restraints on a helicopter.
EXAMPLE. So I have a trasport craft, that is 4 tiles and 6 tiles long. I would have 12 seats, because I have 2 rows on the side with 6 tiles long, or seats. Now my engineers have made a better dropship, that is 6 tiles wide and 12 tiles long. I could fit 48 seats, because I can fit 12 seats on the sides with a 2-seat wide center row. It's the same system passenger planes use. Tanks are 3x3, so in a 6x3 dropship, you could fit 2 tanks. But because tanks are way more heavy than soldiers, the Charlie could only lift one, because it would be too heavy for 2 tanks. Shrike could carry about 3 or 2, and Valkary could fit as many tanks that could fit.