An official ruling on such innovations of such spells would be highly appreciated (shameless plug for help from the sage).
However, I think if I were DMing I would not rule the effect as an insta-kill since there is no real foundation for that in the game. Comparing other types of attacks that would be similarly lethal such as any effect as described as impaling, eviscerating, etc. to the size morph inside a creature suggests that a method of determining damage should be determined. After all, whose to say that you necessarily increase in size in the manner that would enlarge from a central point or that your position does not change while morphing.
If I can be a bit clearer on the last point - let's say you are the size of a flea and currently in a specific 5'x5' square at the direct center. One might argue that as you expand to accomadate the volume of a medium size creature you expand in every direction equally maintaining the central point position. However, there is nothing in the rules to mandate this and it is just as likely that one face of your dimensions remains in its position and the expansion takes place from that plane. Moreover, it is just as likely that the position could shift within that 5'x5' square so that as you expand you never occupy the same space as something else. Moreover, let's say that you are on a 5'x5'x4.999999' block currently in one cube but then as you are ontop of that cube in the shape of a flea and expand to a medium size creature you are now considered to occupy the cube above the one that housed your original position.
Keeping the above perspectives in mind, it seems to me that first, a creature expanding in size will shift in position to fit the available space (if possible). Given the flea in the dragon's ear canal scenario where there does not appear to be enough space, it then seems that the question would be a matter of hardness (in physics/chemistry terms, not game terms). As you expand in size are you "harder" that the dragon's ear (assuming that it has an ear canal). Some might say yes, some might say no.
Let's look at two examples to focus the argument. Let's say there is a construct that consists of pure adamentium and that that adamentium is entirely solid (with no internal empty space). Lets say that this construct has the ability to shapechange and shape changes into something of dimunitive size from its original medium size (just go along with this for the point). Let's place that dimunitive construct inside a sealed sphere constructed of clay - something arguably exceedingly "soft" in terms of my earlier comments on hardness. The construct then attempts to revert back to its original form.
What happens?
One could say the spell simply fails. However, this hardly seems reasonable. Rather I would assume the construct would be back in its original form and the clay sphere would be...well it would be no more.
Lets reverse the materials with the same conditions. So now the construct is clay and the sphere is adamentite. Let's make the sphere sufficiently thick enough to ensure the reasonableness that it can withstand tremendous pressure. So the construct attempts to revert back to its original form. I'd say one of two things can happen - excluding the simple failure to return to original form. The adamentite sphere could be no more or the clay construct would now try to fit its actual volume in a volume that is much to small. This would in a sense be equivalent to increasing the pressure of the clay construct. Rules for this do exist. HAHAHAHAHA - EUREKA. Deep sea pressure rules that explain a character takes something like 10d6 or 20d6 or something of pressure damage every round they are unprotected in the deepsea. So one could say that the clay construct takes this much damage (and vice a versa for the first of these examples with the obvious difference that in the first example the pressure is exerted outward resulting in an explosion and in this example, the pressure is exerted inward resulting in a partial implosion). Also, this damage could be easily adjusted according to the size difference. If one really wanted to calculate this they could deal with all of those pesky physics equations relating force to pressure and then calculate the amount for force in my example to the amount by the deep sea given by the depth and proportianlize the damage.
This of course assumes that one substance is obviously much harder than the other. In the case of the deep sea, water is amorphous, so I would probably also include minor damage - maybe 1/10th of the original - on the construct/sphere respectively since it too would be receiving tremendous force. Of course all of these additions to the deep sea pressure rule are house rules.
So going back to the flea/dragon scenario - was the character encased in a suit of platemail. I certainly hope so - because of dragonscales are any indication to the hardness of a dragon's bones/muscles/interior, most likely the character is going to be taking most of the damage. You could also just rule 50/50 and divide the damage equally.
Alternatively, you could apply the stonewalk/teleport rules that prevent two things from occupying the same space and simply catapult the flea into the astral plane.
Now for the soapbox - I really hate when DMs just rule 0 on something. If the rules don't prevent it let it happen, and come up with some FAIR, let me say that again FAIR, ways of dealing with the issue rather than just screwing the players.
