I was mucking around in ZBrush (v2 is great BTW!) and exported a fairly high res mesh - 12.8 MB (13,508,496 bytes) Vertex Count 131584. I imported it into Poser(PPP & P5 same file size) and then exported it again(all export options unchecked) and the file size for this obj was 20.3 MB (21,303,647 bytes). The same original ZBrush OBJ imported/exported from UVmapper was 14.9 MB (15,674,887 bytes). I have noticed greater differences in higher res meshes - a 38mb Zbrush mesh being 50mb exported from Poser! So why the differing file sizes? I have modelled some meshes that I will eventually bone etc, for use in poser, Is there a good tutorial anywhere for optimising the mesh? Obviously I want to keep the file sizes as low as possible!

The difference are Normals!, Poser export normals information that nobody uses, so the filesize is bigger.

OBJ is a text format, so there's different ways to represent a mesh which can add up to a big change in file size, although the actual model is unchanged. As well as normals/no normals, some applications change the precision of the coordinates (add more digits after the decimal point, or take some off); there can be format differences such as changing the number of spaces between columns; and lots of other things too...

The usual advice is to run an OBJ through UVMapper, which regularises most things. If it's for Poser, uncheck the 'export normals' option. UVMapper Pro has more options, but I'm not near my copy at present so I can't see if any of them might be useful to you.

This is what you must uncheck... A question I have for this is: Does poser recreates the normals when loads an OBJ without normal? If it does not,that means that OBJ without normals (called by cr2 files...)load faster? I've notised Maya doll basic has normals on,will removing them make loading her faster?

Omitting normals should make a file load faster, but I haven't done a comparison to check. Technically, Poser uses winding order to infer the normals. When the OBJ contains a polygon using certain vertices, these are specified in a particular order. If they go anti-clockwise around the polygon, then it will be facing you.