Going back and looking, he was building a bath house to begin with. But I suspect that the hot spring changed everything.
Thing is, water has an absolutely absurd capacity for thermal energy. It needs so much energy to be heated up to that point that some houses in Japan have dedicated water heaters just for the tub. I think that MC was building the bath with the idea in mind that it would only be practical to heat up
x-amount of water at a time because of how much fuel the boiler would need.
But now? Now they have a hot spring and, like I said before, that changes
everything. You now have a practically unlimited supply of hot water just coming up out of the ground, ready to go at all times, and with no further investment of energy in the form of fuel. Which, in turn, means anyone can take a bath whenever they want.
So now he needs to make the tub A LOT bigger.
He should probably just make a whole new partition to the bath house with a much larger tub for the ladies. The men here seem to be in the minority.