The myth that lifting heavy weights will make you big and bulky and that using those cute pink dumbbells will make your muscles long and lean, is exactly that, a myth.
In fact, muscles do not get long and lean from resistance training. And it is also pretty hard work to grow them big and bulky (just ask some really muscly people😳)
You may be surprised to find out that there is no difference in lifting heavy vs. medium or light weights for muscle growth. It doesn't matter how heavy you lift, if you lift enough times to come close to failure (that moment when you have to stop as you are not capable of another rep without losing form or hurting yourself, or you have 1 or 2 reps left in the tank before it happens), you will achieve the same muscle growth and build the same amount of strength.
So you can lift 30 kg only 5 times, or you can lift 3 kg about 55 times and your muscle growth will be exactly the same. The difference is that it will take you a lot longer if you lift light and we are all time poor these days. Another key difference is that lifting light doesn't strengthen the tendons and other joint structures as much as lifting heavier weights and ideally we want to strengthen those alongside muscles to achieve strength.
Muscle size is not the only indicator of strength. Compare weightlifter who is relatively small - like 60 kg and can lift 3 times their body weight, vs. body builder who has bigger muscles but wouldn't be able to lift 180 kg above his head.
Strength training with heavier loads will build the strength quicker than medium or light loads, so if you want to get stronger and not bigger, then you should lift heavy! (lifting lighter will require you to get the muscle bigger to achieve the same strength)
"But there is no way I can lift heavy doing Pilates!" I hear you say... That may be the case with matwork, where we use light dumbbells. If you have ever tried using weights heavier than 2 kg in a Pilates class, you have probably never done it again 😅.
One of the reasons we use lighter weights is to control the challenge level and not invite muscle use that is out of balance with the exercise. (Lifting you arms up with weights that are too heavy make it impossible to stabilise your shoulder blades and keep your shoulders away from the ears, which is the aim of upper core stability exercises) Heavy hand weights create leverage and momentum challenges that can pull you out of alignment; stress your neck, shoulders, and back; and shift the emphasis of an exercise from the core to the extremities. These are just the opposite of what we want in Pilates!
On reformer however, you can really push your load up if you want to gain strength. Don't be afraid of loading the springs for your footwork and leg work. I promise, you won't get massive thighs, but you may just get stronger quicker! And don't be afraid to make the springs so heavy that you have to stop mid exercise for a quick rest. It doesn't mean that you are not strong enough. It means you are building the strength more efficiently!