Default memory capacity

Hello to all!
First of all I want you to know that if it seems obvious that 8GB can be short fast, but it happens from the system stock my Moto available memory tells me they are 8GB dividing 2GB approximate for applications and another 5 GB approximate multimedia , in conclusion they are the 7.64 GB of memory 8gb. but now DIMINISH capacity considerably with custom rom and even uses all memory equally, leaving me only 3GB of storage media. be that the way in which the partition of custom rom is installed on the storage media? previously had a Samsung Advance 8GB and 4GB approximate always left me for multimedia. approximate and 2GB for applications. so my doubt

sorry for my bad english