My 5 month old pup is on Royal Canin but although appears fit and healthy with a lovely shiny coat, seems not as chunky as she was. Is this to be expected at this age or does she need meat added to beef her up. I have tried increasing her Royal Canin above the amount advised on the packet but just makes her bowels a little loose.

In my experience of pups and children, then tend to grow out, then grow up......

Yes Gabby,
All Gsd's go through a gangly stage for about 6 months between 4 to 10 or more months, don't worry about it just carry on feeding as you are (obviously increasing the amount as she grows)
Once they get past 12 months they seem to grow out rather than up.
If you feel she is skinny, try adding tripe, breast of lamb or suet to her food to bulk her up a bit.

But only do this if she is actually skinny, as opposed to very immature.  As a breed that suffers from skeletal problems more than most it is a good idea to keep them on the lean side when growing.  She will fill out with time, this is quite normal, even with my own breed that is quite stockily built. 

Thank God fopr Puppy and Junior classes i say, where at oleast they are only judged against similsr development.

It is quite an education watching the lower classes,a dn then seeing the challenge where the youg ones look it against the winners of the higher classes.