You may have already known some details about Flex that he got those muscles by accident, giving him light powers, which also affects his mane to glow.  
Here, after learning how to manipulate the energy from within through small light balls and other means, he discovered how to unleash that same light energy in full power.
 
This process begins from his entire body heating up and then glows as well as his mane. Muscle mass begins to expand more as light energy begins to break off from it to the point where it all burst out of him through forms of tattoo-like patterns on his chest and arms and wings. This changes his appearance to his true, uncompressed form due to unleashing all the magic, causing the surroundings to become impacted by a supernova-like power.
 
The downside when using this much energy is it will run out fast! Flex will immediately run out of energy, loosing all those light, and reverting back to his ordinary non-muscular pony form . . . as well as destroying everything around him. x3 It should therefore be used as a last resort. xD