Mega-caps are the top of the market-cap ladder. They dominate cap-weighted indexes, so their moves drag the index around: in a market where the ten largest names are a third of the index, an average 3% move in those ten shifts the index by about 1% on its own.
They trade with enormous depth, penny-wide spreads, and deep options chains, so slippage is rarely the constraint. The tradeoff is that they move slowly in percentage terms and are crowded with institutional flow.
Example: a $2.4 trillion company that is 7% of an index rises 5%. That single name adds 0.35% to the index even if the other 499 constituents are flat.
Related: market-cap, large-cap, index