Oozes increased in size - natural armor or no?

[Raphael]

A medium ooze bumped to a large ooze would seemingly gain some natural armor (+2). However, a common feature of many oozes is that they don't have natural armor, just lots of hit points. For instance, the Bog Scum is huge, but has no natural armor. The Carnivorous Blob is colossal,but nary a point of natural armor. A large Gelatinous Cube has no natural armor to speak of as well.

For my game, I want to bump a Crystal Ooze from medium to large.

http://www.d20pfsrd.com/bestiary/monster-listings/oozes/ooze-gray/ooze-gray -crystal

Is there anything which says that oozes don't get the natural armor increase? To do so seems to go against the intent for oozes.

Or just apply the size increase as per RAW?

[Darklord Morius]

The ooze thickens, parts of it crystallizes or accumulates so many impurities, that the formed crust serves as armor.

There you go, natural armor on oozes.