Reduce lightmap size

Hey.

I'im currently having some issues with the lightmap size in my app. One level is 5Mo minimum with a single 2048*2048 lightmap and can go up to 15 Mo with 3 lightmaps.

I've tried many things without success.

Here are my general settings:

As you can see i choosed to set the baked resolution to 1 texel per unit and I play with the scale for each objects which are combined meshes. Here is an example of a piece of wall + floor mesh combined settings:

I tried to lower the values for each type of objects combined to the minimum. For example here for walls + floor below 10 appears some nasty artefacts.

Going for a 1024 * 1024 actually increases the lightmap total size as for 2 2048*2048 it can takes 10 1024*1024.

I absolutely need help for that as for a mobile game 5 to 15 Mo per level appears to be quite huge. Any ideas?

One question also. Does reducing the amount of light probes reduces the lightmap size?