Storage space lost during an UPDATE

I ran the following query on my database:

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update bv_descriptions set description = REPLACE(description, '\\n', '\n');
commit;
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It finished fine but to my surprise when I ran "df" on the server the
drive usage jumped from 44% to 60% which is additional  650MB.

I tried to restart the server but it didn't do any good, eventually I
tried to do a FULL VACUUM on that table and it did the trick and
cleaned the lost space.

I am still confused about what happened... What took these additional 650MB?

I could understand this phenomenon if it happened before I did
COMMIT... But why did it stay after COMMIT too?

Thanks