I'm labouring under the completely unfounded assumption that a crystal's power is based on it's volume, thus the size increase from small to medium is cubed to get the resultant power output.
Thus, a ship twice the size of a skiff will be eight times as powerful. If reality is even remotely like this assumption, I think a larger ship is thus definitely worth investing in.
Though that would normally be an excellent assumption, it's not true. Mostly in order to prevent runaway costs one one end of the spectrum, and clear advantages to a certain class or size on the other.
If the core progress increased by exponential factors, their costs would increase be similar numbers. For instance, the power of a core crystal increases in cost by a cubic while its cost increase by a quadratic, then the medium core would cost 36, and the large core would cost 1296- at ballpark. You might be able to by medium core crystals, but then you run into an interesting problem. If the small core crystal, can block, say 8 light cannon shots, then the medium should be able to block 512 (8^3), right?
You see the problem. Exponents are a PITA to work with and balance.
In general, a crystal will will several multiples more costly, and several multiples more powerful, but the progression isn't as simple or as radical as a doubling of each dimension.
EDIT:
As a PSA, do not vote for you revisions and designs at the same time. Splits votes too many ways. For example, the plan that Jerick is voting for is functionally identical to Taric's, which means I come down to this really messy point where I have to choose between the letter of your votes or the spirit of your votes.
Don't make me make choices. You wouldn't like me when I make choices.