I have a database which grew about 800 MB in the last one and a half
month. This means it doubled in size. I try to understand what is
taking up the space but can't figure it out.

We added a table which logs every changed row of data from an original
table. It does so with the aid of a few triggers. This table is
expected to grow. When I look at the amount of pages in data on this
database, most (about 80%) of the growth is in this table. I figure if
I multiply the amount of pages with th pagesize (4096) I get an
estimate of the size the database should have grown. In that case the
table grew around 60 MB, but the increase in filesize is nearly 800 MB.

Does anybody have a clue where I can find the remaining 700+ MB?

Note: Backing up and restoring the database doesn't make the file any
smaller. The backup also grew from around 500 Mb to 1300 MB