If the small parts and the larger polygon have a common ID, you can use the Dissolver transformer with the common ID as the "Group By" attribute.

thanks @david_r for your answer , No ,they don't have a common ID ,no attributs ,just geometry

If you don't have a common ID to dissolve on you'll need to create some rules to determine which polygons should be dissolved together, e.g. you might decide that everything under a certain area should be dissolved to the adjacent polygon

thanks @ egomm but where can I find this option?

It's not a single option. You would have to find all polygons less than 100sqm, find which polygons neighbour them, choose which to merge with, assign a common id and then dissolve. Depending on the complexity of your data it can get quite involved

A simple example

dissolve_small_polygons.fmwt