Will encrypting hard drive reduce disk size?
When I open Diskutility, I see a change for partitions that are encrypted. They each have two entries, a parent and child. The parent is the normal partition, and inside is the encrypted child partition. As the encryption is always done by the OS, it is software encryption. 

The resulting encrypted child partition is smaller than the parent. As I had several encrypted partitions, I was able to compare them using the numbers in Diskutility. I can only give you the numbers from this one example, so I don't know if they are similar on other systems. For this computer, encryption takes 320MB data. One partition (root) uses 4KB less for encryption. Disk Utility reports for the parent root partition that 4096 Bytes are free. The other parent partitions have 0 bytes free. Why this is - I have no idea. 

Below you see the data in bytes for all three partitions: 150GB, 300GB, 850GB. 


(A)Partition   (B) Parent      (C) Child      (D) Difference  (E) MB
1 (root)       149484212224    149148663808   335548416       320.00390625
2 (external)   300060016640    299724464128   335552512       320.0078125
3              849726693376    849391140864   335552512       320.0078125


Difference D2 - D1

335552512 - 335548416 = 4096 = 4KB


As I use 256 bit encryption, these numbers may be different for 128 bit. If anyone can confirm this, that would be nice!