Spoken like a complete noob. a.) The OP did not ask about liquidity he asked about size comparisons - Here it is in bold print for you. The spot/cash bond market is far larger than futures. How big is the UST cash bond market vs UST futures? b.) Informed & pro traders care a great deal about the instruments underlying cash volume exchanged, if it's not large enough one hedge fund can fling the price around wildly in the direction they desire. c.) Commodity futures will never be worth more than actual spot, its so far from reality as to be laughable.
Let me quote from OP, he was pretty clear I think: That's true for most derivatives and not true for the truly liquid futures. Liquidity begets liquidity, in general, and cash tends to be following the futures most of the time in things like index futures. I.e. tail starts wagging the dog. You see these types of relationships well by looking at small-scale (milliseconds and sometimes second-scale) lead-lag correlations, it becomes pretty clear. Where do you think delivery squeezes come from, then? PS. while it's fashionable here, I am not trying to belittle your knowledge or anything, I am just throwing in some factual information
Just an FYI, calling @sle "a complete noob" is, trying to put this nicely, laughable. I'm actually a little embarrassed for you because you are a generally intelligent well meaning guy here, and that comment doesn't paint the most positive picture of your situational awareness.
I am a complete noob at many things, so there is nothing wrong with that. To be fair, I think @comagnum misunderstood the OP and I came off a bit argumentative in my initial response to him.