Either the questioner is near illiterate or a genius. More likely the former.... "How" means "by what mechanism?". If one is referring to the Metric System, one must query just what the Metric System entails. Due research would reveal it refers to a quest for a universal system of measurements. It would oppose the measurements of the day which might refer to the length of a monarch's appendage (along with whatever exaggeration which might seem appropriate) with no regard to to the vagaries of age, usage or imagination might entail.
Further research would show the frustration involved with shipping a product more than a "mile": or whatever measurement system the seller might invoke (my mule refuses to go any further) so the system would further involve weather and possible hormonal activities. Such activities are extremely difficult to quantify.
One quickly realizes the system of measurement not only involves the origin of the item but also the origin of the buyer, and more importantly, the attitude of the seller, which we must assume to be "Whatever I can get, as long as I feel I am coming out ahead".
Taking these factors into consideration reveals yet another area of far greater importance, which entails the eternal bureaucracy. Any transaction involving the transfer of wealth in any form alerts the sensory apparatus of whatever body which considers itself to be entitled to a portion of the transaction. These bodies, which pre-exist any and all forms of civilization, legitamize their life-form through shrewd and ingenious manipulation of whatever form of communication the present society might encompass. In short, whatever the common citizen might utter, the bureaucrat has some phrase of indistinct, yet ephemeral sounding utterance, guaranteed to touch the sounding board of yet other bureaucrats who will garb themselves in strange garb and headgear and gather in hushed surroundings uttering odd sounding noises and even stranger sounding phrases which invoke past personages of indistinct background but in the recording of fellow personages who were in similar condition at the time, have attained the patina of "TRUTH".
Those who depend upon commerce to support their existence will invoke their versions of "Truth" to not only support their own existence, but justify the spread of their version of "Truth", so long as it adds to their share of the booty.
The answer to the original query therefore must be: Measures are flexible and dependent upon the needs - real or not - of those who are able to influence the measurement. In short, when a bovine relieves itself of used and excess nourishment, whatever is produced is measured according to the conditions of the day. That is, the purveyor of such will endeavor to use whatever means are at hand to maintain the weight and mass while the purchaser will attempt to allow the product to lie in the sun, allowing evaporation to reduce both weight and mass so as to lessen his cost. Meanwhile, the bureaucrat will create a justification for taking a majority of the end product through a list which will be invoked by his followers which justifies his existence. Should either buyer or seller become unhappy at such action, said bureaucrat will empower those of his employ to not only take total control of the entire product, but to punish those who dare to question such action by relieving them of all which they posess.