When you alter in size there is a multiplier that gets applied to you. You can find the rules in this regard
here:
"Bigger and Smaller Creatures: The figures on Table: Carrying Capacity are for Medium bipedal creatures. A larger bipedal creature can carry more weight depending on its size category, as follows: Large ×2, Huge ×4, Gargantuan ×8, Colossal ×16. A smaller creature can carry less weight depending on its size category, as follows: Small ×3/4, Tiny ×1/2, Diminutive ×1/4, Fine ×1/8.
Quadrupeds can carry heavier loads than bipeds can. Multiply the values corresponding to the creature's Strength score from Table: Carrying Capacity by the appropriate modifier, as follows: Fine ×1/4, Diminutive ×1/2, Tiny ×3/4, Small ×1, Medium ×1-1/2, Large ×3, Huge ×6, Gargantuan ×12, Colossal ×24."
Also the faq you are linking is for a different kind of transformation penalty. That one isn't specifically just for size changes it is actually more aimed at type chances (specifically ones that normally can't wear armor). For example wild shape giving a boost in strength (because you transform in a specific animal) but still staying medium. Means you still keep the original encumbrance. While if you chance in size you do gain the size increase bonus mentioned above which CAN alter the encumbrance. (They kinda started to mess slightly with the entire transforming info a lot lately)
But if a humanoid just uses enlarge person you just become bigger and can carry double the weight. And as far as I know not all items on you count as gaining double their weight (not 100% certain about this one tho thought just equipment). But if the item multiplier isn't x2 for everything like it is for weapons and armor. It might be the size increase lets you carry more.
And also the added +2 STR can alter the encumbrance. So be sure to check all factors just in case