How much storage space do indexes and keys take for an SQL Table?

I have a table which is predominantly filled with NULLS and takes up 10 GB which was shocking since probably 90 to 95% of the table is NULLS. I copied this table over to a new database on the same SQL 2012 instance using Select * Into.... but the copied table takes up about 70 MB. Checking them against each other, they seem to have the same data, as expected so it's not like data is missing. Is the disparity in size a result of not generating a script to copy indexes, triggers, keys, etc? Is there a way to optimize the table to reduce space? Thank you,