can I drop P-index of E-fact table after compression?

Hi, experts!

Maybe this question has been asked a lot of times, but I could not find the appropriate answer.

After an infocube has been compressed, its DB size increased in 2 times!

The reason is that the huge P-index was created. This index occupies the same DB space as the E-fact table! So P-index and E-table together are twice bigger than F-table was...

We have a lot of uncompressed infocube data in BW. So I am afraid of compressing it, because DB space is small.

Please explain me:

1) why this P-index is so huge?

2) can I drop it after compression has finished? is there any negative effects?

Thanks in advance.