It is "double" muscle mass... myostatin regulates muscle growth. The mutation prevents myostatin production and so the muscles have no signal to stop developing.
It's a recessive mutation so both parents need a copy of the mutant gene for it to express in the children. (homozygus)
There is a second mutation that does something similar, but it blocks the uptake of myostatin rather than the production. It's equally rare.
Bully whippets are the same height as normal whippets if that helps you picture the overall size of them.
Here is a link to better pictures to demonstrate the size difference.
Anzenberger Agency - Bully Whippet
Again long term health problems are still undetermined, as bullies are produced by breeding parents with single mutations. I'm not sure if bullies can reproduce naturally.
The cattle for example can't give birth to their massive offspring and need to deliver via c-section.
wa:do