If you have 4 ints of 6 bits then you can only store 1 byte in each of them, which ends up being 4 bytes total of course.. which is 1 int, so you arent getting any extra space. Ohhh!! wait, its compression because you can store 4 number values (1 for each of those ints) but it doesnt actually compress, it just makes it so youre able to store 4 different numbers in the size of 1 int in the end, whereas if you only had 1 full int you would only be able to store 1 number because you cant split it up. I get that now, cool, but that means youre limited (like you said up there) to the size of your number because you only have 8 bits to use.. so no big numbers.
For example an int is 4 bytes, if you have 4 ints of 8 bits, it will compress it into a single int of 4 bytes.