Where 3D printing went once STL's limits got too obvious. It stores each vertex once and refers back to it, and carries colour, materials and print settings that an STL has nowhere to put. Being a zip of XML, a 3MF of the same model usually lands at a fraction of the size.

Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop a 3MF file above and choose afterwards.