It depends how you build them largerer magazines can be more efficient in space so a 20 HS magazine can have 1 HTK in which case it will hold more missiles than 20 size 1 1 htk magazines of the same tech.
Ummm I just checked this in the game, and it does not seem to be the case.
I'm at feed efficiency of 90%, so a size-1, 1HTK magazine has a capacity of 18 (20*90%). A size-20, 1HTK magazine has a capacity of 360, which is 20x the size-1 capacity. (It also has a research cost of 1000, which is 20x the research cost of the size-1 magazine.) The material cost is also 20x larger. The reason that this scaling works is that 1HTK essentially means "unarmored".
This means that, with the current rules, size-1, 1HTK magazines are MUCH superior to larger 1HTK magazines - 20 size-1s have 20x the HTK of 1 size-20. I think this is actually a bug which could be fixed in 2 ways:
1) Make the choice be HTK/HS, so that a size-20, 1HTK/HS magazine had a HTK of 20. This would make the size decision neutral.
2) Subtract 1 from the HTK scaling. What are currently 1HTK magazines would become 0HTK (like small fuel tanks, they're destroyed but don't absorb the hit). This means that unarmored magazines would all have 0 HTK, and the scaling works again.
Note that once you put armor on the magazines (raise the HTK above 1), the economies of scale kick in - it's cheaper to armor a big magazine than a small one, due to the lower surface/volume ratio.
John