system
Given the following post, respond as if you're commenting on Reddit:
human
Why are batteries different sizes? What is the difference between using four AAA vs. two D batteries?

Wondering after looking a some flashlights of mine. One uses a couple D batteries while some use AAAs and some use AAs. Do the size differences have to do with voltage, or “amount” of battery? Could you theoretically change something that runs off a D to run off two AAs?
gpt
AAA, AA, C and D cells are all 1.2 to 1.5 V when new. The difference is the amount of current each can deliver, and for how long. Bigger cells can deliver higher current and can deliver more amp\-hours. Alkaline AA are roughly 3 Ah each, but D cells are roughly 15 Ah each.  So two AA \*in parallel\* may be able to power something designed for one D cell, but for around 40&#37; as long. Five AA in parallel are equivalent to a single D cell. 

Combining cells in series leads to voltage differences which most circuits are incapable of handling. That is, you can't put two or more AA in series to replace one D cell.