Why are those miningfields that small? You need to stick to that gasminer before you can actually mine.
All the other ore fields have a normal radius, why can't helium-3 have that?
Because the Gas Miner process the regular helium found in the field, and purify the He3. You "mine" what those purify (if I'm correct).
I do not see any problems with that, I've been mining He3 without a problem almost since I play Disco. It gave me no difficulties (in fact, it works even better in 4.87 than in 4.86).