While it’s a natural occurrence, some still claim their muscle mass is due to steroid injections due to the large muscle development. 

The Belgian Blue are however not born with that extreme muscle, but start developing that muscle at 4 to 6 weeks old. 
Belgian Blue Beef outclasses all other beef breeds in carcass yield (up to 80%). When used in cross-breeding programmes of other dairy or beef breeds, it increases carcass yield from 5 to 7 % compared to the maternal line. 
Extensive research has shown that Belgian Blues possess a gene which suppresses the production of myostatin, a protein that normally inhibits muscle growth after a certain point.  

Pure Belgian Blues carry two copies of this gene; in cross-breeding, one copy is usually transmitted and serves to increase carcass weight in the offspring of a cross-breeding program. 

As a result the Belgian Blue carcass produces a high yield of saleable meat with a boning out % in excess of 85%. 
European comparisons between the Belgian Blue and Charolais found the Belgian Blue to have a higher muscularity, milk yield and daily gain. The Belgian Blue animals were also older at sexual maturity. 

There is a common misconception in the cattle breeding industry that the Belgium Blue has a poor calving record and that many are calved via caesarean. This only occurs in pedigree breeding on stud farms when purebreds are calved to purebreds which is mainly done in Belgium. 

Internationally the Belgium Blue is used as a terminal sire for meat production. When used as a terminal sire the results show that this breed is easier calving when compared to breeds such as Simmental, Charolais and even Friesian calves born to Friesian cows mated to Friesian bulls. 

 Results of studies conducted in Belgium show that the carcass characteristics of the Belgian Blue were expressed in the cross-breds whose commercial value was then substantially increased. The observation explains the growing interest for the Belgian Blue breed as a terminal sire, its carcass composition and its growth potential. 
– cattlesite.com