Our modern high-output LEDs are marvels of efficiency, however they still put out a whole lot of heat. You don't notice it so much in many lights, because the light was specifically built to keep the LED cool - the LED sits on the thickest part of the flashlight assembly, which acts as a heat sink and draws heat away from the LED. The flashlight feels a bit warm, letting you know the design is working.
Say we actually do put an MC-E in a Mag Solitaire host, we give it a power source with the proper voltage and current, and fire it up - the light comes on normally, and moments later we notice the light getting hot. Very hot. SCALDING hot, hot enough that you can no longer hold it.. not long thereafter, the light begins to dim, some smoke comes out of the head of the flashlight, darkness, and that would be it, one completely fried flashlight.
It would fail because there's not enough of a heat sink - a tiny flashlight doesn't have enough metal mass to wick heat away from the LED, so it just gets hotter and hotter until it melts down. That's why all the brightest flashlights aren't very small, you need the size to manage the heat.
You do still have some impressive size-to-output-surprise flashlight choices; one perennial favorite is the Fenix P1D, a 2.8" x 0.8" 1x123 light that puts out 180 lumens:
https://www.fenix-store.com/product_info.php?cPath=22_65&products_id=354 , and Lummi features the Raw (1.8" x 0.7", up to 200 lumens) and Wee (1.3" x 0.6", up to 100 lumens) models:
http://web.mac.com/Lummii/Lummi2/Lummi_Home.html
