"Is putting a small weed plant in a big plant pot bad for it?"

Yes it is.
That's why we go to all the trouble.
It's also why we do the minimum impact transplant.

If the root ball is too small for the pot the soil can get waterlogged and go stagnant.
Then the benefits of a large pot become detriments.
They stunt, then die.

Not saying it can't be done, just that it is extremely difficult to do properly.
I'm way too lazy to do it over, so I up-pot in 4 stages and even then water very carefully.
1 pint, - 1 gallon - 3 gallons, - 5 gallons.

This summer, I'm trying for trees and have some 10 and 15 gallon bags waiting for the eager Sheila's.

Aloha,
Weezard