macOS installations shows less space than I have I cleared my system to install the BigSur beta:

So I have 62 GB free space.
But when Im in the installation process its shows the following:

Just 28 GB free space. How can I fix this?
when I look up my HDD via finder it shows me:

So that 34 GB should be purgable. But how to delete this stuff since my recycling bin is empty.
I found a solution:
I just created a new APFS parition and reserved as much space as possible. Then the space which was shown as purgeable was deleted automatically. After that i removed the partition again and had all the free space.