Well if they have overcrowded the mining environment that does not allow the smaller outfits to compete for the amount of coins available to mine. Then yes they are controlling how much is available across the board. Essentially hording all the available power to mine those coins available.
I have the same thought, if they are mining major part of bitcoins, they have a lot of coins in head, if they hold all of it, they can control the price and for sure they will mine for years and years, none can compete with it. In some way this is good for all holders.