The shields listed for the Idris and Javelin read "Capital" while the shields for the Polaris read "Large." Can you explain this difference, and why the Polaris has large instead of capital shields?

In short, the Polaris shields are not as strong as those found on the Idris and Javelin, but this is a trade off in return for its relative speed and firepower. Also in terms of scale, both the Javelin and Idris are quite a lot larger than the Polaris. It's also worth remembering that shields need to be placed in the context of their deployment; the Idris and Javelin are both larger and slower, and so in general they're going to be exposed to concentrated fire more often and more frequently than a Polaris - especially perhaps by capital-size weaponry. When ships are designed with actual components taking up internal volume, you can't just fit a super-sized everything in the hull, so just as with real-world engineers designing warships, often you try to find an efficient and effective balance of equipment and bulk. Shields also require power from power plants in order to work, so you might get better results from running a smaller shield generator at peak efficiency rather than a larger shield generator with only a little juice. Since a corvette is smaller than larger capital ships, you can't just pound for pound put the same reactor, generators, avionics, etc. as a larger ship across the board without bloating the design.