Yes, there is more force exerted on the sheets than the halyards as a general rule...but if the halyards stretch, it will affect sail shape much more than if the sheets stretch, since the sheets are constantly being adjusted anyways...while the halyards are not.
If your halyards stretch, the sail starts to bag in strong wind conditions, which will exert more force against the halyards, and stretch them more than would light wind conditions, it will be more difficult to keep the sails flat and de-power them. In the same conditions, if the sheets start to stretch... you won't even notice more likely than not.
This is why wire halyards were popular, and why high-tech, low-stretch lines are so popular on racing boats. Also, the high-tech lines, like T-900 or Endura braid are generally lighter and stronger than their polyester analogues like StaSet or even StaSet-X. Using the high-tech lines means you can often go with a smaller halyard, reducing weight aloft a bit.
3/8" StaSet-X has a tensile strength of 5500 lbs. 4.5 lbs/100'
8mm T-900 has a tensile strength of 7300 lbs, but is both lighter and thinner than the StaSet-X. 3.2 lbs./100'
8mm Endura Braid has a tensile strength of 7000 lbs, but is lighter than the T-900 at a slighty reduced strength. 2.7 lbs/ 100'
Does it make sense now??
Finally, it is generally easier to replace sheets than it is to replace halyards. So if you have stronger halyards, the chances of them breaking is smaller... if you break a sheet... you generally don't have to go aloft to replace it...