Don't they ever.....
I tend to think of Sunnie as a good bit bigger than Danny even though only 7 lbs. separates them. I'll look down at her when she's alone and wonder where the small dog came from....and I have a couple of pictures of the two of them where Dan looks bigger than his mama.
When Danny's the only dog in sight, he sometimes looks larger. I'll actually do a "rib check" sometimes to make sure it's just my imagination working overtime.
I think Sunnie looks bigger in my sig pic than she does, relative to Dan, in person.