Egg temperature for meringue
The reason they're supposed to be warm is that cold eggs tend to curdle, and de-emulsify. This is only really an issue if you're adding a bunch of fat (because otherwise you don't get an emulsion). They should also get a bit bigger, which may explain why they seem to shrink more when you fold things into them.

You can always try adding a little cream of tartar in order to stabilize them, which might help.