It's a good thing to ride different horses and deal with horses that are more challenging - that's how we gain experience and grow as horsemen. Sometimes it can be frustrating when a horse truly challenges you to accomodate yourself to his/her idiosyncrasies, but learning how to be flexible and adapt to different horses is an invaluable skill.
That being said, moving up to "more horse" in a lesson, especially for a younger or less experienced rider, should be done with some sympathy from the instructor. When I was taking regular group lessons as a kid, if I was handed a new mount, the instructor would say "How about we put you on Flicka today? She's a princess on the ground, but she can be a bit balky and heavy on your hand once you get going, so prepare for that and be sure do XYZ, like you learned on Trigger last week, to set her straight. We'll talk more about that once you're mounted up." I suspect that Maggie will be a horse that can teach you a lot if you are shown the proper tools to work with her successfully.
I, too, disagree with arm strength being the key to dealing with her heaviness, or that "she needs to lean on the rein" to be in balance. Your core muscles are of far more consequence in rider strength than arm muscles. If she is leaning on the rein, she is not in balance and is relying on you to do her job. That can be fixed by not bracing the rein against her leaning, combined with pushing her forward so that she can't dump her weight into your hands as easily - something that your instructor *should* be able to help you with. If he/she insists that leaning on the rein is a good thing and didn't just misspeak, well, it's definitely time to look into another instructor.
To me, it sounds like perhaps it was just a bad day for all involved; a large class of mostly younger kids, at least one fussy pony, etc., can all put an instructor on edge. That being said, if this experience is similar to your other lessons, not a bizarrely bad day, perhaps it is time to look into a different lesson barn. Another option would be to take a private lesson or two on Maggie so that you and your instructor and iron out your difficulties without taking time from the other sudents in the group setting.
Lesson stables run the gamut - some are excellent facilities with caring, knowledgeable instructors with a variety of suitable ponies that the instructor can and does pair up with riders as appropriate lesson-mates from one day to the next. Others are less stellar, for one reason or another. If you do opt to look for lessons elsewhere, I highly recommend observing a few lessons (most instructors don't mind if you are out of the way and aren't disruptive) before signing on to get an idea of the instructor's style.