Size and strength are not the same thing, even if one intuitively implies the other. So it goes with weather and storms. During springtime, when the air in the Northern Plains is usually somewhat low in humidity, it is common for smallish thunderstorms to form in the dry environment. Most of the time, these little thundershowers do not produce severe weather. However, looks can be deceiving.
Under certain conditions, such as when air temperatures are very cool and the wind is blowing a different direction in the higher parts of the storm, it is possible for even a fairly small thunderstorm to develop a strong updraft with enough rotation to generate large hail or even a tornado. Likewise, sometimes midsummer storms may grow higher than 60,000 feet and only produce rain, thunder and lightning. With weather, looks can be deceiving.