I assume you are referring to Gustav Adolphus and his relative strength during the 30 years war. I'm afraid I'm not qualified to answer the second part of your question. Now my source is dated, but I would cite three major reasons (there are of course many factors at work, and Gustav Adolphus was a brilliant General making some major changes in how wars where fought):

• Sweden had a conscription system earlier than most other powers, most swedes where called into a state militia, that the regular army drew from. This in an age when most armies where raised either from mercenaries or press gangs (an oversimplification). This meant Sweden could field a larger army for its size than its neighbors. This was of course in addition to troops raised in Germany and England.
• Rich allies, Sweden got large amounts of money from the England France and the Netherlands, who all wanted to keep the Hapsburgs from becoming to powerful. This of course enabled Sweden to pay all foreign troops she hired.
• Light artillery. Gustav Adolphus made artillery far more portable than earlier, it was practically considered a peace of siege equipment. G.A. pushed for light more portable cannons that could play a larger role in battle, breaking formations and killing horses.

Of course this relative advantage didn't stay around forever, England started recruiting larger standing armies with the new model army, and france perfected it with the levee en mass, but more importantly Sweden couldn't project that much power without money from her allies, so when there money went elsewhere Sweden was just a relatively poor nation with a limited population again.

Source: mainly Gustav Adolphus by Theodore Ayrault Dodge

edit: formatting, and added source