Assuming you are talking about WW2, its important to understand, that they didn't fight everybody at the same time. French had lost the war before the US or the Russian even joined the war.

Also if you consider Germany, Austria and Czechoslovakia by the beginning of the war with Poland, they are actually had quite a high amount of population and during their whole war with France/Britain they had the full support of the Soviet Union and were able to buy resources. So France was defeated and Britain fought of by Germany with the cooperation of the Soviets.

Now, by the time Germany start the war with the Soviet Union, they have absorbed France and a lot of resources and manufacturing again, plus capturing huge parts of Eastern Europe and of course allies in Italy, Hungary, Finland. At the point of its maximum expansion the Thrid Reich plus allies was quite large and had a huge population, that they did try to exploit.

Another important part is the Britain and the US had to deal with Japan joining in 1941 as well.

After 1942 Germany basically just defended and if you have a large empire and lots of people with a good industry, you can play defense for 3 years even if you are outmatched on all fronts. The Allies faced the massive challenge of landing on the coast as well.

To elaborate a bit on the response by u/panick21, Germany in 1939 cannot be called a small country by any means, at least when compared to other European nation of the time. After the annexation of Austria and Sudetenland, population of Germany, already 69 million strong before 1938 numbered almost 80 million people. This was much more than the population of Poland (32 million), United Kingdom (40 million) or France (38 million) and constituted 63% of the population of United States (128 million). Even USSR, often quite fancifully portrayed as a country with innumerable reserves was a home to roughly 200 million people, i.e. only 2.5 times more than Germany (all statistics are for 1939).

Now, contrary to quite common narrative, Germans were not really fighting the war on two fronts until the Operation Overlord or late 1943 at the very earliest. The war began with the invasion of Poland that has been successfully finished after a month, partially due to the support of USSR. When it became apparent that western Allies won't retaliate, Germany followed up with the invasion of France through Netherlands and Belgium that also ended with the surrender of the invaded country. Only then they followed with the aerial attacks against Great Britain that was a first wake-up call, as Germans proved to be unable to make any serious advances. It is true that the war in the Northern Africa was fought concurrently with the invasions in Europe, but Germans committed only limited forces to that theater in the form of German African Corps, with the bulk of fighting being carried by the Italian allies. This allowed Germans to focus on the Operation Barbarossa in 1941, as with majority of the forces readily available to attack USSR and western flank relatively secured (United Kingdom was correctly considered unable to launch any counteroffensive at that time, and United States did not join the Western Allies yet). And even then, with the casualties ratio far higher than the ratio of manpower on the Soviet side (in many battles of 1941 and early 1942 Soviets suffered 4-5 times more casualties than Germans compared with their forces being only 2.5 times higher or slightly more when we include irregular militias), Germans were able to make significant advances on the Eastern Front. However, between the economic support Western Allies rendered upon USSR, shortages of raw resources, defeat of the Italian Army in Africa, Allied presence in Italy and eventually also the invasion of Anglo-American forces in June 1944 the opposition was staggering, what led to Germany's defeat less than a year later.

Also, please note that the Germany in 1930s were a totalitarian state able to focus its industry and economy on the military production at the drop of the hat, what was often criticised as unsustainable by many contemporary economists and industrialists. Thus, Germany was able to produce vast number of modern weapons, artillery and ammunition as well as combat and transport vehicles, what further increased their militaary strength. In addition, morale in German army was very high, with some historians suggesting that it was caused at least in part by the limitation of the Germany army stipulated in the Versailles Treaty, that forced Germans to do the best with what they were left with and focus on the training, drill and instilling high morale and spirit of meritocracy among officers and camaraderie among soldiers, what has been later extended to the entire army to a good effect.