Let's look at, well, first one I pulled off the SRD, a Fire Giant.
BAB +11
Grapple +25
Str 31 (+10), Dex 9 (-1)
He's Large, so he gets a -1 penalty to attack.
So, the question is, what should the touch attack to initiate a grapple be at. By the RAW I guess it ought to be +20 (BAB+Strength modifier-size), which is pretty much going to overcome even a slippery halfling's touch AC (say, Dex 20, Dodge feat, gives a touch AC of 17 for a small person). Does, however, adding Strength to this attack make sense? If you changed it to a Dex modifer, he'd have a pathetic +9 touch attack, which wuld be more in keeping with the slow but strong idea of giants (although still standing a good chance to grab our slippery halfling).
But then, a Large giant isn't *that* big - he's 12ft tall, which is only about 4 times the size of the halfling - it's hardly trying to catch a fly, or a puppy for that matter.
If we up the size difference - a Huge Storm Giant,
Str 39 (+14), Dex 14 (+2)
BAB+14/Grp+36 (Size mod +8)
Now try the negative-grapple size-based modifier, he's got a base +6 chance to initiate a grapple. If he's doing a spot of giant-wrasslin' with a fellow Storm Giant, he's only got to beat a touch AC of 10. If he's trying to grab the slippery halfling, he's got to beat AC 17.
If you used the inverse of the size-based grappling modifier (in this case 'twould be -4) then you would have an initial touch attack of +7