Datruth, lets use a little logic here

This is how i see it;
You know how much strength you have, you know the max weight of your inventory.
You have a limited number of slots in your inventory, and if you had 1 item in each, you would be under weight but un-able to carry any more items. When you double up items it increases the weight you are carrying, but you are still limited to a set number of different items.
When using the sac, you add more slots to your inventory, increasing the number of different items you are able to carry. The more sacs you carry the greater the number of slots you have available to utilize.
Yes I understand you probly already knew that. Having said that, here is where the 'rules', 'speculation', 'guess' or whatever you wanna call it comes in.
I dont believe the sacs were intended to be magical(as to increase your carrying capasity), I believe they were intended for expanding the inventory slots, to allow a greater number of different items. When implimented, I think the exact values were enterd incorectly, giving them an unstable limitless capacity.
In order to be sure you dont drop all of your inventory, I believe you must keep your overall weigth, sac inventory included, equal to or less than your shown max weight capacity. Any amout over that will be unstable.
Untill we get an official description of the sacs abilities, overloading them would be a bug exploit.
ATTENTION DEVS: If this is incorect, please let us know

Maybe this is a good question for the December "ask a PS question" thread.