New installation does not use full disk

[Heinrich Schuchardt]

I am trying to install Kinetic Kudo on a RISC-V SiFive Unmatched board. I selected the attached USB as installation target and "use full disk". This is the proposed layout:

  Storage configuration [ Help ]
▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀
  FILE SYSTEM SUMMARY ▴
                                                                             █
    MOUNT POINT SIZE TYPE DEVICE TYPE
  [ / 28.347G new ext4 new LVM logical volume ▸ ]
  [ /boot 2.000G new ext4 new partition of local disk ▸ ]
  [ /boot/efi 1.049G new fat32 new partition of local disk ▸ ]

  AVAILABLE DEVICES

    DEVICE TYPE SIZE
  [ ubuntu-vg (new) LVM volume group 56.699G ▸ ]
    free space 28.351G ▸

  [ Samsung_SSD_960_PRO_512GB_S3EWNX0J51671 local disk 476.939G ▸ ]

Why is so much free space left instead of using it for the root partition?