It's even quite logical if you think about it. If he was charging with one hand, he'd only have one output into the deathball, making for a slower charge. Two hands allows for a quicker charge (twice as many outputs) which would only logically be used if he was going to want to fire it quickly, meaning it would still be relatively small compared to a planet-destroying version. Smaller in size usually computes to smaller in damage and blast radius as well.
Can Goku move with his secondary Spirit Bomb miniature thing? I can't really remember at the moment, having not used him in a while since the Brolly pack came out. =\ If not, I think it's only fair that both of these attacks should be similar, while changing Frieza's primary Death Ball to static, of course. If you want to have Frieza able to move with the secondary, then perhaps Goku should be able to as well with his secondary Spirit Bomb. Just thoughts...