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I'll be blunt - I think Mugs is heavy.. puppies are *supposed* to be skinny and lanky at that age. Maybe not to the extreme that Mocha was, but ribs SHOULD show, they should be skinny! Since you are used to Max's heavy weight, it probably doesn't look wrong to you, but boxer puppies should be "stocky" - even pups with a decent amount of bone go through skinny stages (look at Odeya's Jack). I think that may be why Kat mistook her for a boy - the heaviness seems to give her a masculine look?
She looks fine to me. She has some substance, but she doesn't look overweight to me.