Increasing output current of the inverting voltage doubler charge pump circuit, implemented with MAX1720
It isn't the transistors that are limiting the current, it's the capacitors. A capacitor of a given size (and a fixed supply voltage) can only transfer a certain amount of charge per switching cycle, and that charge, multiplied by the switching frequency, becomes your output current.

The only way to increase the current (keeping everything else constant) is to use larger capacitors.