Talk of Russia having total operational control of Crimea does not imply that they control Crimea. No, Russia controls the major transportation nodes and is prepared to accept reinforcements to actually control and defend Crimea. Russia has the backbone built to flesh out their invasion.
Yes, Ukraine's military is weaker than Russia's. But Russia's military doesn't have that many effective troops. Perhaps 100,000, as Strategypage has noted in the past.
Russia is a big country. Could they really deploy all of that effective ground force to a Ukraine war? I'll guess they can spare half (and yes, it is a total guess). And then add in the same number of army-like Interior Ministry troops (the kind that fights in the Caucasus)--another guess.
In theory, Ukraine could mass their power to drive on Sevastopol--counting it as a victory just to cut off communications to Russia via Kerch and deploying air defense and anti-ship missiles around Sevastopol to interdict those methods of supply--while holding Kiev in force and mobilizing reserves to reinforce a thin army screen in the east.
My counter-attack scenario works better if the Ukrainians can maintain support in the east and deploy mobilized reservists in the urban areas of the east.
And there is a big question of just what does Russia have in Crimea:
The U.S. originally estimated that 6,000 Russian troops were dispatched to Crimea, but Ukraine's mission to the United Nations said Monday that 16,000 had been deployed. That stoked fears that the Kremlin might carry out more land grabs in pro-Russian eastern Ukraine.
The statements are not necessarily incompatible. Several years ago, Russia had 13,000 personnel in Sevastopol--mostly non-combat troops. They had a force of naval infantry there--at least a thousand but I thought it was 3,000. Now I don't remember my source for the latter, so assume the former.
If Russia reduced their garrison in Crimea as they reduced their military, one could assume that Russia had 10,000 troops in Sevastopol, including a thousand naval infantry.
Then, if Russia moved in 6,000 more troops by air and via Kerch or from the sea, you could say Russia dispatched 6,000 troops to Crimea and that Russia has 16,000 troops in Crimea.
Russia is reportedly gathering armored vehicles on their side of the Kerch Strait. So more is coming.
And Russia needs more if it comes to a fight. Seven thousand combat troops--mostly lightly armed paratroopers and naval infantry, it seems, is not much of a force to hold Crimea. The 9,000 support forces could probably man the Sevastopol perimeter and other static positions, but they aren't a major combat force to fight an invasion.
I don't think a military option is ruled out for Ukraine despite talk of massive Russian superiority because in the theater in question, Ukraine should be able to gain local superiority for a while anyway.
And at least make Russia fight for their conquest.
UPDATE: RFE/RL cites a figure of 11,000 Russians starting out in Crimea, including 2,000 naval infantry. The rest are support personnel. Russia moved 6-7,000 troops into Crimea during the last 5 days.
Special forces airborne forces from Moscow have been identified as well as troops from Russia's southern military district more recently. I assume those are forces crossing at Kerch.
That's not a lot to control Crimea.