That's the same argument as saying food is only 4% the economy. Try taking food out of the equation and see how the rest fares.
What you say is true though, short term, but I think I might still rather be in Russia's position in terms of fossil fuels than that of the US, when we look at things long term. What Russia has the option of doing - which is also being discussed elsewhere in this thread - is using those resources domestically, to grow/develop its own industries and (currently dwindling) population. In fact that's what they're being pushed to do by the sanctions (as they have been developing their agriculture as a response to previous sanctions). I've been really really skeptic of the sanctions (to put it mildly) since the beginning. They will always find ways to sell their resources (if they want to).
The argument by throwaway71271's is pointless and made only as a distraction from the initial point that Russia is a petro-state heavily dependent on oil export - without those EXPORT revenues the entire state is at serious risk of collapse.
Obviously EVERY economy has many critical components that are yet a small percentage of the overall economy; your mention of food is an excellent example. These are not the question here.
The only relevant point is that the USA is not even close to dependent on it's exports of oil and gas. In fact, NatGas exports were entirely banned until recently. If all USA oil & NatGas activities were suddenly done on a nonprofit basis at cost, the economy would only shrink marginally. If Russia did the same thing, it would likely collapse on a timescale of single-digit months.