However a number of futures markets used to have rules that the size of the market positions of speculators weren't allowed to exceed the size of the market pos

However a number of futures markets used to have rules that the size of the market positions of speculators weren't allowed to exceed the size of the market positions of the buyer / sellers; this was to prevent the speculators driving the market rather than the buyers / sellers. The most famous of these is probably the position limits imposed by the Commodity Exchange Commission (forerunner of the Commodity Futures Trading Commission) in 1938 on the commonly traded grains. In most countries these limits have been removed over the last 20 years or so. The debate on the necessity of such limits is still ongoing...