Tempered underground under high pressure and heat, its body is harder than any metal.
– Pokémon X Pokédex
Steelix already looks top-heavy, Mega Steelix looks even more top-heavy, with its giant face and small body. It gains these colourful crystal things on the side of its body, as well as floating crystals. It almost doubles its weight upon mega evolving (from 400 lb to 740 lb).
It gains the ability Sand Force, which increases the power of its Ground, Rock, and Steel moves by 30% in Sandstorm.
Stat-wise, Steelix’s ridiculously high Defense gets even higher to 230, making it one of the Pokemon with the highest defense (tied with Mega Aggron and Shuckle). It also gains more Sp Defense and 40 more Attack, making it more offensively viable.