It is difficult for a tier-1 prime broker or bank to be competitive in the FX space because they do not cater to smaller market players, essentially restricting themselves from that market flow. Therefore, regardless of the fact that major banks are able to take huge volume, they do not have the capability to provide liquidity the way prime of prime, and exchanges do.
Economies of Scale
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Only these market players have the incentive of aggregating prices into a best bid best offer feed, which provides quality in depth of market, and makes it easier to fill orders. The Tier 1 Prime brokers do have a major benefit though, which is cost savings through economies of scale; however, the benefits of these cost savings are typically only reaped by larger firms.