The log reflects the structures maps has generated internally (in memory) so far in the current calculation. When you restart it, it does load the structure that were output to disk, but it does not automatically regenerate all the structures that were internally generated in the previous run. (This is to avoid a long delay, if one just want to quickly re-fit a cluster expansion with slightly different settings.) As you run longer, maps starts regenerating its internal list of structures, hence the maximum volume grows.
Note that you can force maps to recreate, at initialization, the previous internal list with the -g=[volume] option. In your case -g=44