It happens when one requests generation of only some MWMs present in the source OSM dump.

E.g. Ile-de-France region (Geofabrik link) is split in OM into 8 mwms, one of them being Paris.

When I generate all 8 mwms together (--contries=France_Ile-de-France_*) everything looks normal and e.g. Paris' mwm takes ~19Mb, Hauts-de-Seine ~16Mb, etc.

But when I use the same source dump to generate just the Paris (--countries=France_Ile-de-France_Paris) the resulting mwm is 65Mb and area covered is waaay beyond original borders, but smaller still than the whole source region.

Compare a "normal" Paris, an "overblown" one and the total coverage of all 8 Ile-de-France mwms:

Furthermore, when I generate Paris together with e.g. France_Ile-de-France_Hauts-de-Seine then both of them come out bigger (Paris 38Mb, Hauts-de-Seine 43Mb), but smaller than the whole region still (note a big "hole" right to the south of Paris, there is not hole in a bigger Paris from above).

So the resulting mwm sizes and borders are hardly predictable.

CC @vng