OntarioSheltie
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Mikos is a tall sheltie at 18", and he weighs in at 30 lbs, but his sister is only 13.5" and a little feather weight! In a litter of 5 Mikos is the giant, his sister the smallest and the rest fall in between, so lots of variation even within families.
Yes! I have two over sized shelties and one that's the appropriate sheltie size. Ginny, who is 14" and should weigh 14 lbs (she's a bit chunky right now at almost 15 lbs.) has a brother in the same litter who is closer to 19-20" tall and weighs in at 32 lbs. Ginny looks exactly like a miniature version of her brother.

I'd be sooooo shocked if I found out Miko's brother was even bigger than him now!! He was such a dainty little guy! Mind you... Miko wasn't exactly a bruiser back then either. He was "in size" all the way up until about 16 weeks and then he slowly started to creep off the chart. Even with being sickly and thin... he just got taller and taller. Then once his tummy issues were resolved the muscle tone appeared and he filled out in the giant sheltie oaf he is!! 
Especially next to confirmation shelties and many of the agility shelties...