The file sizes depend of your export settings and of the model complexity.
Certain functions, as the fillet function, or the text function (based on a high vectorized sketch) will strongly increase the size of your exported file. By default, the export in .STL have satisfying standard settings.
If your file is too heavy to be imported on Sculpteo platform, you will have to use the mesh design workbench. Try to reduce the level of precision directly in Freecad, or if your model integrates many fillets, curves or sketches base on text (or vectorized graphic), try to export it in a more appropriate format as the .IGES or the .STEP.