As an owner of THREE standardbreds (im assuming stdbd means that) I can tell you that this is perfectly normal... well on a scale of the horse is getting enough in its diet etc (but like you said you're not getting onto that, and nor will I!)... it could do with her build... but I've found with a few Standardbreds that they take longer to develop in the growth timeline! My wee 14.2hh mare only really seemed to stop growing (both physically and mentally) when she hit eight... before then she was like a baby... and didn't even have the mature mentality that I had expected in earlier years... but that could be to do with her personality. She had started to fill out around six years.
My friend's Standardbred gelding is I think seven, and he was still bum high at five and six years of age he started to fill out... I own his 4yr old half sister and I believe she's gonna be along the same lines, just not as bad ;)... my opinion is... they excel in their growth early on in life, for use in the racing industry... they gain all their basic form before some other breeds, and then the growth slows down and isn't completed until they're around eight. I could be wrong, but it's just my own opinion here. I think five years isn't enough time for everything to settle into its own place etc either.. I wouldn't be too worried... and it seems that you're onto it with the feeding agenda! Besides, like someone said, I too class a horse who is five years old a baby!
♥ Seoul Searchin' for the Lovebug ♥