Disks reports size of the thinpools including the metadata size (API docs: http://storaged.org/doc/udisks2-api/latest/gdbus-org.freedesktop.UDisks2.LogicalVolume.html#gdbus-property-org-freedesktop-UDisks2-LogicalVolume.Size). So for lsblk output you need to use size of both "test-tpool_tmeta" (4194304) and "test-tpool_tdata" (33554432).