The best I can find right now is in the DMG on page 110 under "Extreme Cold":

Whenever the temperature is at or below 0 degrees Fahrenheit, a creature exposed to the cold must succeed on a DC 10 Constitution saving throw at the end of each hour or gain one level of exhaustion. Creatures with resistance or immunity to cold damage automatically succeed on the saving throw, as do creatures wearing cold weather gear (thick coats, gloves, and the like) and creatures naturally adapted to cold climates.

(Emphasis Mine)

Another thing that I found is in one of the functions of Boots of the Winterlands, described in the DMG on page 156 (emphasis mine):

You can tolerate temperatures as low as -50 degrees Fahrenheit without any additional protection. If you wear heavy clothes, you can tolerate temperatures as low as -100 degrees Fahrenheit.

This second piece of information is very minor, however.

At any rate, I don't see any price listings on adventuring gear tables or what have you. So it's definitely there, but it's probably just a minor detail that can be dealt with through narrative.

The AC changes, to 13

In their human form, the PC weretiger with 16 DEX wears the studded leather armor, giving them 12 + 3 (= 15) AC.

They polymorph into their tiger form.

Any equipment it is wearing or carrying isn't transformed.

As you note, the armor does not transform to fit them, so they do not benefit from the human-shaped armor. It is the DM's discretion whether the armor is destroyed, damaged, or just hangs off the weretiger's new form uselessly after the transformation.

Since they are not benefiting from the armor any more, they can no longer use the AC calculation (12 + DEXMOD) provided by the armor. As Solias notes, in the Player Characters as Lycanthropes box on MM page 207, AC modifications in different forms are noted for other types of lycanthropes, but not for weretigers, so their AC is now simply the basic 10 + DEXMOD = 13.

Equipment is not a statistic

Your confusion seems to be centered around the following passage:

Its statistics, other than its size, are the same in each form.

And this is true.

If the weretiger were wearing a Ring of Fire Resistance which it took off and dropped exactly as it transformed, it would not still have resistance to fire in its tiger form, despite "resistances" being a statistic. This is because fire resistance is not a statistic of the PC, but something provided by the ring to the PC.

The same is true for the AC calculation (12 + DEXMOD) provided by the studded leather armor - it is not a statistic of the PC, but something provided by the armor to the PC.

Jeremy Crawford has tweeted to clarify that equipment is not an inherent part of monsters in the context of a similar question around the Shapechange spell - the same is true of PCs.