When Santa opened the door in Frosty the Snowman, some of the puddle must have evaporated already, so how was Santa able to reconstruct Frosty from the puddle? I assume the reconstructed snowman after Santa put the hat on it was smaller than the original one the children built earlier?
It's 100℅ conservation of energy, the puddle may have began to evaporate, and thus, simply changed form. Into a gaseous state.
The magic did not discriminate between Frosty puddle and gaseous Frosty, and so all of Frosty was reconstituted, into snowman Frosty.
So, I guess "reconstituted" Frosty would have ended up being about the same mass as before.
Santa said as much, as he explained there is no need to worry, as when Christmas snow melts it comes back as rain and when kissed by December winds returns as Christmas Snow...