My springer pup is 9 months old, and the amount she can grow in a week is unbelievable!!
At one stage we thought we'd picked up a sausage dog, she looked soooo long

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There's a springer pup that is about a mth younger than mine that walks passed our house and I must say I was shocked at how small he was compared to my pup. But within a few weeks he is catching up and it won't be long before they are the same size. (both working types)
I did find with my pup that changing her diet seemed to help with the weight, Beta didn't seem to be filling her up, whereas as soon as we changed to Arden Grange she took on a whole new shape, it might be coincidence but personally I feel that the change in diet made her thrive.
When thinking about the size of your pup, the best comparison would be to her mum and dad - if they weren't big springers it's unlikely yours will be.
A smaller springer sounds like pure heaven to me, at least your arm won't get broken when lead training (as any larger springer owner will agree, they can pull like no-ones business:rolleyes:)