I doubt that DBCC UPDATEUSAGE will change this. This command could be needed in the dim and distant past of SQL 2000 and older versions.
Why the difference is that big is difficult to say, without a full repro of how you arrived there. But one possibility is that you loaded data with a minimally logged operation, and only a few rows at a time. Every new transaction will start with a new extent.
You say that this table used to be a partition in the old table. So another possibility is that data has been deleted and updated, leaving lots of empty space in it.
In any case, it is not the partition switching that cause this difference - it was there in partitioned table as well.
You could rebuild the indexes to compact the table.