No, each MXS represents a frame. But if most of your objects are static, then export them as an MXS, import it as a reference. Then the only things that will be taking up hard disk space when exporting the animation will be the camera movement and any object(s) you have in the scene that are animated. Saves you from having a 300MB MXS for each frame, and instead have 1 reference which is 300MB, then several 10MB or so MXS files per frame. Lots less to upload.
You can optimize the workflow and frame export size even further by using the Alembic format for animated meshes. Exocortex Crate has a nice Alembic plugin for Softimage. Simply export your animated object to abc format (you choose the frame range you want to export and all of the meshes animation will be contained in a single abc file), then import it again in your scene using the MXS Reference functionality of our plugin, only instead of pointing to an MXS file, point to your abc file. The advantage of using an abc file (compared to exporting them to one MXS per frame) is that it can store animated meshes very efficiently. If your animated meshes contain many hundreds of thousands of polygons then it will make sense to first export them as abc files. On the other hand if your animated mesh is a few thousand polygons then probably you can go ahead and leave it as is in the scene and export as "usual".