In index rebuild process my data files get full and then empty also in huge amount....while Sort_in_tempdb=On

In index rebuild process my data files get full and then empty also in huge amount....while Sort_in_tempdb=On

any idea?

ex:ALTER INDEX [d_1f05349780000502] ON [dbo].[dmc_coll_relation_s] REBUILD PARTITION = ALL WITH ( PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = OFF, IGNORE_DUP_KEY  = OFF, ONLINE = OFF, SORT_IN_TEMPDB = ON )
GO

Thanks