I agree with Renoman that you don't know what associations your pup has with stairs - she may have been hurt on them previously, or an impatient owner may have scared her by pulling her down.
I know that going *down* stairs is much harder for dogs than going up - it took us about two months to get Coco to come down the stairs on her own. She would walk up fine, but we encouraged her by putting a bit of treat on a step about three down from her, then another three so she came down gradually.
It sounds like at her age and size, encouraging your dog to manage the stairs on her own may be preferable all around to trying to carry her on them each time. Good luck!
And I also think big breeds may have more difficulty because the treads are that much smaller in relation to their body (although I've never had a toy breed and I imagine there may be a converse difficulty there!). My friend's male wolfhound was about two before he worked out how to come downstairs on his own, very distressing when you've got a HUGE dog stuck upstairs...
