As of 1.13, the Giant Mushrooms use the multipart model feature in their blockstate files, meaning every msuhroom block consists of 6 independent models now, each being either an inside (skin=false) or outside (skin=true) block face. The inside faces/models show the porous mushroom inside texture, the outside faces/models show the mushroom skin texture. This new behavior makes mushrooms extremely flexible and theoretically allows them to generate in any shape and size.

I found an error in current mushroom generation though: The Red Giant Mushrooms generate with 5 of the 9 top blocks (the center block and its 4 neighbours to be specific) having one "true"-value too much, they should only have one red side texture (except for the center block) and the red top texture with skin=true specified, but there's an additional side face showing the outside skin. This is not visible from inside or outside the mushroom in the vanilla resource pack (unless you break the top corner blocks), but a resource pack using connecting borders for the red skin faces makes this error easily visible from inisde the mushroom.

The skin=true|false values for each mushroom block can be checked by pressing F3. Added three screenshots to clearly visualise this issue.

UPDATE - 18w06a:
As of this snapshot the new world generator was added. The 5 top blocks of red giant mushrooms now each generate with even more superfluous skin faces: The 4 side blocks now have 4x the value "true" in their blockstates instead of 2x, the center block even 5x instead of 1x for the top face. Added another screenshot to show how it looks now with a resource pack adding borders to the skin faces. The side blocks of the mushrooms remain unchanged and are all generated with the correct blockstates. After the release of 18w06a, there was a short interval from snapshot 18w07b to 18w08a in which the mushrooms were reverted to 5 top blocks having one red skin face too much.

Note that huge red mushrooms generated with bonemeal always show 1 red skin face too much, while naturally generated mushrooms have 2 too much, which is another weird discrepancy I've noticed.