Jose, this - indirectly, too has to do with the sensor size. Because the sensor is small, the image focused onto it is also small. As a result, Leica and Pansonic were able to develop a "mini-superzoom lens". In the film world a comparable lens would be huge, its size needed to focus on a plane 35 mm in size, the size of the film. These large lenses contain a dozen and more elements (up to 18 or more) to compensate for flare, reflections, and absorption physically caused by the light traveling down a larger lens. The more elements, the more light is lost - up to 1/2 to 2/3rds of an f/stop. The longer the zoom, the lower the T-values, the light transmission index of the lens. A regular prime lens with fewer elements results in little light loss inside the lens itself and is able to maintain its brightness.
Back to the sensor size. The small sensor means a more compact lens. Its conpact size results in less flare, reflection, and absorption physically caused by the lens itself, and no need for twice the number of light absorbing elements. The FZ-1 lens, I think (spec book not handy), contains six elements(?), as opposed to 12 to 18 elements found on a regular, large telephoto film lens.
Smaller sensor = extremely small zoom lens size = less distortion = fewer elements (six as opposed to eighteen) = lens able to maintain brightness throughout zoom range.
Oldud - Yes! Van Riper loved the FZ-1. The FZ-10? Not so much. Why make the darn thing bigger? Compactness was one if its biggest assets.