hi amber, welcome to FOH.
by all means, it can't hurt to speak to a nutritionist - even if you are doing pretty well, it's nice to know where you can tighten up a few things or add in some more variety.
pilates and yoga are brilliant as they are stretching types of exercises - and stretching as you know by how it feels, elongates the muscles as you're doing them. by increasing that flexibility and strength, you'll be able to begin working on how you want them to be.
i don't know if there's a specialised way of doing it, but i guess doing the above is one way.
cardio - walking or running - is another way to work on reducing fat within the muscle mass.
one thing that i do that kills my legs, but is nice and i feel gives me leaner legs is to put some ankle weights (those velcro on things) on and then do some floorwork. this is kind of old school, but one thing i don't have is horrid legs - frightening belly LOL but nice legs. i've done those floor leg lifts (and also standing leg lifts) for years. sort of a throwback to when i used to do ballet i guess (as a kid)... things like squats and lunges will work on the different muscle groups within your legs, as will doing leg presses and that kind of thing. it just depends on what you're comfortable with.
when you say they are "quite large & shapeless" what do you mean? are they fat and flobby large & shapeless, or just non-toned.
i doubt that spin classes will give you bigger legs, but you will likely build up great muscle strength and a bit more tone through your calves which probably won't be a bad thing!
hope this helps somewhat!
kaz