They didn't move into a small apartment, its a stand alone villa which in Seoul, SKorea would be very pricey.
It still looks smaller than the condo they were in
They just wanna be close while the baby is small, makes things easier. I'm sure once the kiddo is older they'll want everyone to have lots of space, but foe now, having the baby close by makes it easier to keep an eye on him~ Also after what they went through, I'm sure they both want to keep each other close.
They could be limiting space, a lot of parents do that to avoid having to baby proof the entire house and keep the child close in case the baby starts crying. Also the author themselves didn't emphasize more on the surroundings. Except for a couple panels, all are of them close up. If you look at the last panel you can see the stairs going up, it could be bigger the artist just seems like they wanted to focus on the couple and kid.
So you mean to tell me they moved out of that big azz condo into this small apartment because they are a family now? Come on now LOL