An update - I know why there are three .ser files - it is because my Blender scene has three objects!
Anyway, I reduced it down to just the landscape plane, which is an OBJ file of 672690 bytes, but the Mesh Serializer plugin converts this to a .ser file which is 1311204 bytes, as before, which is too big to load.
So the problem still remains, in that the Mesh Serializer has generated a file too big to load into my android app. Perhaps it should warn users, or not allow this to happen? Or is there a way of configuring it to split a big object into multiple smaller objects? Or should I just do that in Blender first, and export multiple smaller objects into a single OBJ file which will then result in multiple SER files?