i think she looks like she is well on her way, and agree with the other reply.
for muscling, trot trot and more trot. and when you think you are done, trot some more. build up in increments so your horse has time to condition (start with just 10 - 15 min depending on how fit she is now) and then added 2-4 minutes per workout every other to every third workout. and stop cantering. for a few reasons.
1) canter burns more calories than it builds muscle, while trotting builds more muscle while burning less calories when compared to the canter
2) horses develop a better canter by doing better trot, not by cantering more, so by taking a break from cantering, when you come back to it, you will be amazed with the improvement
3) lots of bending and changing direction and working on a light rein contact will do wonders for also helping her balance and relax and again help her canter in the long run
4) all that trot work will build amazing topline and muscling
pictures to prove it:
Sky, aged 4, just off the track going through "letdown"
Sky, age 7, after a few years of remuscling (notice neck still very thick/upside down):
Sky, age 11, schooling at a show:
Sky, age 14 (this year) widest he has ever been - at a hunter pace/horse park: