This is a common concern, especially for people with fine or thin hair. The Mirrolla filler is formulated for home use with a light texture that is easy to distribute along the lengths without weighing the roots down. It is a leave-in concentrate, not a heavy oil, so it is meant to be used in small amounts.
The key is how you apply it. The product is designed for clean, towel-dried hair, and it works best when you focus on the lengths and the ends — the areas that are most damaged. Avoid applying it to the scalp or the very top of the head. This way the roots stay dry and the product goes exactly where it is needed.
The filler creates a thin protective film on the hair that reduces friction during combing and blow-drying. This film is what makes the hair feel smoother and look glossier, but it also means you should not overuse the product. If you apply too much, any leave-in product can make the hair look heavy or greasy. Start with a small amount, distribute it evenly, and add more only if the hair still feels dry or rough.
For fine hair, an even better approach is to use a smaller dose, focus on the ends, and avoid the roots completely. The product is not designed to add volume, but it will not flatten the hair either, as long as you use it sparingly.