Is it? That they have more volume is clear, but I don't think that's a good indicator of liquidity. On BTCChina for example, there are  about 3k coins on the bid side 10% down from the current price point. Bitstamp is about the samw and for Bitfinex it's 8k. From my perspective, the exchange able to absorb the biggest dump (of whichever currency) while having the price move the least is the most liquid.