Well, the obvious answer is size. Minis are smaller, and generally have only one light source (one bulb). There is no "official size" where a mini becomes a full size pendant. What you select is really based on what you want for your kitchen. Of course, you will need more minis to project the same amount of light, so it will cost more to hang them (or take more work if you DIY).

There are three primary considerations for pendants (or minis) over an island: light projected downward, light projected upward, and appearance. Pendants with opaque shades will only project light down, so you will need other light sources above the pendants (such as canisters). Mini pendants, even those with clear shades, will generally project less light in every direction. You want sufficient task lighting on your island, without shadows or glare.

Pendants without lenses (open at the bottom) may glare, because if they hang at about eye level or just above, you have this bright bulb just a couple of feet from your eyes. This can be worse with smaller pendants because they use smaller, but extremely bright lights, like xenon. Some of these are like staring into a flash bulb.

As you can likely tell, we went through all these considerations. What we ended up wanting for our island, which is smaller than yours (6 1/2 feet X 2 1/2 feet) were two full size pendants, with lenses, that would hold at least two bulbs each, and had clear shades. We ended up with Restoration Hardware Clemson units (from the outlet store, luckily):

These are 14 inches across at the bottom, and take two normal bulbs each. They throw a lot of light downward, and around the room, which kept us from having to put as many canisters in the ceiling. And since I did all the installation work, that saved me time and money. Plus, we just like the looks of these big, bold pendants more than having three smaller ones. That's just personal preference, however.

bumping - would 3 pendants of 6.75 inches on an island of 8x4 feet be enough?

thx

use balloons and string to simulate