Yes, larger tanks are more stable, but interconnected tanks are not. Each still has it's own water volume, and there's also the problems of potential flooding, tank draining, etc. Now sumps are a whole different aspect. Yes, it does increase the water volume. The difference, with a sump, you are usually running it with a higher volume water pump. Thus, the turnover rate in the sump is quite high. It's like water going through a canister filter. So yes, it does add water volume, which in turn increases stability. You don't want to interconnect 2 10G tanks for a total of 20G and have a turnover of 10x-20x on each, your fish won't like you.