I’m not sure they are undersized, the reveals look splayed to me, I’d say the windows are correct externally.The window looks way undersized, but the fitter should have made good. Is this a diy job gone wrong i.e. an error in measuring?
Blup
It’s a bit hard to see from the image.How should these holes be filled after new window fitted?
The guy fitting said they would just put expanding foam in... to me these holes are too big for foam.
Whats the official proper way to do it?
Do you mean on the RHS of the window?It looks to me like a zoomed shot and there is a clear gap, but the OP can clarify.
Blup