I don't have the Donegal but I have a lambswool roll-neck from last season that is a bit oversized. I got my usual size, and yes, it tends to be a largish Medium. However, this style of sweater is supposed to look that way. It's not meant to be slim and small. It's supposed to have a vintage nautical profile. It's supposed to look big, in other words.
I see what you're saying, but I found the length of it too long too, and because I have a small frame already, it feels a little like if I'm drowning in it. I'm happy if it works out for other people, cause it's good value right now.