If you have multiple objects in your blender file (even ones that are toggled to be inactive), exporting as an .obj will still include them unless you select the “export selected” box in the export window, making the export larger than you expect as Garen suggested above. Rule that out first.
If there’s nothing that seems to be the issue, I know that Studio imports meshes slightly differently when a face has a large number of vertices attached to it, which if your mesh isn’t triangulated before exporting, be sure to do that and re-upload it.
And finally, cutting up a mesh that’s over 10k tris in most cases is really not wise. In the large majority of cases, you absolutely don’t need that many tris for anything purposed for Roblox. You should definitely consider simplifying your topology and using a texture for any smaller details.
If none of these are applicable to you, please edit your first post to include images of your object (something you should have included in the first post regardless).