I have a question about cells. If I have a sub cell that’s rated for 3000 Mah, could I buy a smaller cell (physically smaller) of the same capacity to make a pack lighter for this hobbico electric plane?

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43, I hope you come back to check on your thread. If you post a question, we're going to post the reply, not email it to you. That's how forums work. It's a community thing. Emailing you the answer only helps you. Forums are about helping everyone, keeping the answers archived so others can benefit from the knowledge in the future.

Anyway, the answer is yes to your exact question. However, there are other factors you haven't taken into consideration. A cell's physical size is what determines how many Amps it can produce. The battery you choose for your application not only has to have enough mAh to give you a reasonable flight time, it has to have enough bulk to withstand the Amps being drawn by the motor. A battery that's physically too small will heat up quickly, and power will drop off drastically in only a few seconds.

I like to use a clown car vs school bus analogy. Pack 16 clowns in a VW Beetle. Load 16 clowns in a school bus. Now, light both vehicles on fire. The 16 clowns in the school bus file off in a matter of a few seconds, none the worse for the wear. The 16 in the clown car thrash and struggle to get out before being burned to death. Some of them don't make it, and most sustain injuries. That's how it is with batteries of the same capacity, but different physical sizes.