Houses in my location never really stop 'settling'. Look up the word subsidence if you are not familiar with it. My two hundred year old home's door and window frames look like trapzoids.

Probably a lot of it is attributable to seasonal changes. Trim moves when the humidity changes. Foundations that were not deep enough when the house was originally built could heave from frost, (esp. this winter in the northeast!) and any new wood structure brought into the renovation is going to acclimate and, well, _settle_.
Things that would be cause for alarm would be diagonal cracks in the walls, new cracks in the cellar/foundation walls, and doors and windows that do not close when they closed a month or two ago. That's indicators of potentially serious problems, and not to be dismissive, having seams in trim crack or spread open in winter is not a threat, but an inconvenience; it's not structural, it's a cosmetic thing. If the gaps get worse an do not close back up in summer, you'd have to have it evaluated.
Could the builder have done something to prevent or minimize the shrinking? There are steps that can be taken, but not having been there, I can't tell you. There are material choices, construction methods, and painting techniques that will minimize movement and shrinkage, but even if every step was taken, when the humidity drops from 95% in August to 5% in January, it is going to manifest somewhere. Do you have a humidifier?
Casey

Hi

I was just talking about this with my sister after noticing the same thing in her 250 year old house. She said that the cracks appear every winter in the plaster and disappear again in the spring (or at least become not noticable).

Yep, I've got the same thing with caulk joints around the moulding in a 150 year old house! The first winter after I painted, I was disappointed & a bit PO'd, but after 15 years or so, I've gotten used to it.

Hi, Conn123

We had that situation and hired a structural engineer for a consult. This was a while ago, but if you can afford it, that is one way to find out if what you are seeing is normal or if it is a problem that must be addressed.

"I was just talking about this with my sister after noticing the same thing in her 250 year old house. She said that the cracks appear every winter in the plaster and disappear again in the spring (or at least become not noticable)."

These are often refered to as 'working cracks' that open and close as the wood in the building swells and chrinks between cooling and heating seasons.

There are some claims by various companies of flexible material that can move with the substrate and keep the crack invisible year round, but every time I have tried them they have failed.

They bulge during the swelling phase when the crack closes, or pull away from the material when it shrinks, opening the crack again.

I have even had cracks that were open near the floor of houses close at the floor and open near the ceiling when layers of roofing were removed.

Nothing like taking a few thousand pounds of shingles off the roof to change the loading on the structure.

A common problem with molding is using inexpensive 'painters caulk' to fill gaps between the trim and the wall.

Better quality caulk has at least a chance of staying attached.

I prefer the siliconized acrylics that are not as expensive as full silicone caulks (that do not accept paint) but often double the really cheap stuff.

Around $4-5 a tube is what I typically pay.