Short answer: balance between realism and performance (UPS).
Now let me explain in detail.
Yes, it would be ideal in realism, since the weapon needs not store energy but just take directly from grid system.
BUT, in game update mechanism, I used mainly event "on_player_ammo_inventory_changed" to do the charging, which triggers when the amount of magazines of player ammo slot changes. This means if magazine size is set to one, the charging call would be made EVERY shot! Each charging call has to scan for every equipment item on grid and try to take energy from it. This takes some update time.
It may not be apparent if you play singleplayer only and play on a powerful computer. But on multiplayer servers, when multiple players are fighting at the same time and with high fire rate weapons (e.g., chainsaw, laser machine gun), the update time MULTIPLIES, potentially causing performance hit like laggy and choppy gameplay.
Having a magazine size means the update is called only when the whole magazine is consumed. Magazine size of 25 (chainsaw) means the charging calls consume only 1/25 of original update time. I scaled the magazine size with the fully-upgraded fire rate, to roughly an update every 1 second (instead of every 0.04 seconds...)
So I guess it is a good compromise?
PS: Some similar mod used event "on_tick" instead for this kind of update. But I am never a fan of using on_tick event. Using this means the mod is keep consuming some update time, even if no one is fighting. (I guess fighting takes less than 10% of total play time for most players?) Thus, using on_tick is not as effective IMO, and makes the script unnecessary complex (i.e., bug-prone) to distribute the calculations to multiple ticks in case of multiple players.
