OK... i wish i had not to enter this technical details, but...
The THC starts to be metabolized immediatly after it enters the body. The metabolization of it makes you high, and also decomposes the THC in another substances.

So, lets suppose we have weed with 1g of THC. (I know its and absurd amount, its just for the example).
Lets suppose too, we smoke it in a joint, in 10 hits (each hit with .1g).

We take the 1st hit, and 0.1g of THC enters the body, and starts to be metabolized, which means its amount in the body start do decrease. Lets suppose, also, that 10% of the THC is metabolized in the time interval between 2 hits. (I have no idea about the real rate, its just an example)

So, when we take the 2nd hit, we will have only 0.09g from the 1st hit (as 10% of it already are gone), plus 0.1g from the 2nd hit, making a total of 0.19g after the 2nd hit.

In the interval between this 2nd hit and the 3rd hit, this amount will decrease 10%, due the metabolism. So, when we take the 3rd hit, there will be 0.171g from the 2 first hits, plus 0.1g from the 3rd hit, making a total of 0.271g after the 3rd hit.

Repeating this process, when we finish the 10th hit, which is the time when the amount of THC will be the maximum, there will be only 0.651g of THC in the body, as the remaining 0.349g were already metabolized during the time we smoked the joint.

Now, if we smoke this 1g of THC in just one big hit, all this 1g will be present into the body in the time you finish this big hit.

So, there is a real difference between the maximum levels of THC into the body when we smoke fast or slow. And thats why the big hits make you more stoned.

Also thats why the amount of THC needed to OD with weed must be smoked in a small amount of time (some minutes if i remember well). If you smoke slowly, there will be time to the THC be metabolized, and it will not reach the levels needed to OD.