My theory plays in the fact that Universe 6 Saiyans grew in the peaceful Planet Sadala instead of Planet Vegeta. It's been stated that a Saiyan undergoes a MASSIVE growth spurt when they reach teenage years. But what if this growth spurt is a response to threats just like the Oozaru transformation? It's possible that the Universe 6 Saiyans have their own growth spurt but haven't reached the age yet or the stress needed. I think the reason the U7 Saiyans are so bulky and massive is because their Saiyan heritage comes from the aggressive ones which required their body to quickly build up muscles for impending threats and keep their tails and Oozaru forms as a last survival resort. But what about Goten and Trunks? They lost their tail because they were born with power levels greater than an Oozaru so their body treated it as a recessive gene. However, the genes required for a growth spurt has been there laying dormant. But they grew up in a mild environment so why grow?? Keep in mind that these kids fought Majin Buu and all kinds of things added with their training that might've been sufficient for their bodies to register this as a "threat". In the case of Cabba and the gang, I think they haven't reached their quota of beatings yet probably more Zenkai boost. Remember that their species already lost the tail and is flooding with S-Cells from their mild-mannered Saiyan traditions which makes their body more smaller in frame and stature as their power levels are just enough to make their bodies think "Yeah I'll be okay". However, the next canon appearances of U6 Saiyans might be different as they've been subjected to a lot of beating in T.O.P and pushing their limits might've triggered the genes they need so we will most likely see a Cabba standing at the same height of Vegeta in the future.