Why does imported txt table consume so much space
There is a metadata storage overhead for columns in SQL Server that you do not have to pay for in a flat file. Using compression can reduce this overhead. From Row Compression Implementation:


  It reduces the metadata overhead that is associated with the record. This metadata is information about columns, their lengths and offsets. In some cases, the metadata overhead might be larger than the old storage format.


Try compressing your table with row or page compression and see if that reduces the space required in SQL Server.

ALTER TABLE table1 REBUILD WITH (DATA_COMPRESSION = ROW);


In my experience, compression can both improve and degrade query performance. It depends on the data and the query that you're running. It's possible that you'll need to make a trade off between space and query performance.