There's much more to it than that.

1. Not all oil is created equal. Most especially, oil has different qualities of hydrocarbon lengths (longer == heavier, more tar-like) and contaminants ("sour" == more sulfer). Light sweet is the optimum input for most transport fuel needs (petrol, kerosene/jet fuel, diesel).

2. Not all oil has the same extraction costs. In particular, Venezuela's oil has a much higher cost of extraction than Saudi Arabia's.

While there's a very strong tendency to attack Venezuela for having a failed socialistic / command economy, possibly with some merits, a much more direct issue is that Venezuela benefitted from oil which in the past was easier to extract and bring to market than its offerings today, combined with a long history of extensive domestic market subsidies of basic goods, including both energy and food (which, if you think about it, both reduce to "energy).

The same story has already played out in large part through the nations affected by the Arab Spring. Whilst we're not used to thinking of Egypt and Syria as large oil exporters, the truth is that they had some local oil extraction operations, enough to provide exports and some hard cash (of a useful quality: US Dollars, which had the added benefit of not affecting the value of their domestic currencies in foreign trade). Tunisia, Libya, and Yemen are also being affected by falling oil exports (the first two were where the Arab Spring originated, Yemen has had ongoing conflicts).

Russia is watching its cash reserves fall, also hit by a soft market for global oil. This has some strong parallels to the fall of the Soviet Union itself, in the wake of the late 1980s oil glut, then driven by increased supply-side dynamics as North Sea and North Slope, as well as offshore Gulf of Mexico extraction came online.

Saudi Arabia has the optimum product -- light sweet crude which flows (fairly) freely from the largest oilfields and most productive wells in the world. But depressed prices are hitting it as well, and its financial status has been discussed for the past year or several -- as with Venezuela, Saudi Arabia buys domestic tranquility with its oil export cash. As with much of the Gulf region, it also faces some of the most rapid population growth in the world (oil exporting states are a curious exception to the usual rule of Demographic Transition).

There's another interesting situation, one I've just run across a standard economic treatment / term for -- inverted or "backward-S" supply curves. Usually applied to labour, this also seems to fit the situation of natural resources. Where market prices fall to close to the marginal cost of extraction, the tendency of suppliers, especially those with fixed costs to meet, is to increase rates of extraction. This is quite typical of oil boomtimes (Titusville, PA, Spindletop, TX, and the East Texas Oil Field most especially). In the case of the latter, oil prices in the US fell from $1/bbl to $0.13/bbl, and then further to $0.02/bbl, before the governors of Oaklahoma called out their respective national guards, and in Texas, the Rangers, to sieze wellheads at force of arms to constrain supply. The result was the implementation of US production quotas, in effect from 1932 - 1973, under the US Department of Interior and the Texas Railroad Commission. It's a fascinating bit of economic history.

The size of reserves doesn't matter so much if the costs of extraction, or quality of product, aren't in your favour. Venezuela is somewhat cursed with riches, in that respect.