curious vacuum full behavior

I have 2 identical systems A + B.
B being a clone of A.

The table pg_catalog.pg_largeobject was identical on both systems: 300GB in total size; 100GB bloated.

I did following on A:

? vacuum full pg_catalog.pg_largeobject;
(using the default maintenance_work_mem of 64MB)
It took around 45 minutes and increased the diskspace by around 125% until the vacuum had been finished.

I did following on B:

? set maintenance_work_mem = '256MB';

? vacuum full pg_catalog.pg_largeobject;
This took around 5 minutes. I don't know if the diskspace ever increased.

I was really surprised.
Is there any explanation on this behavior?
Is vacuum full heavily using on-disk sort areas if maintenance_work_mem is too low?

Postgres Version 9.6

Thanks, Markus