The big difference comes from the big contrast in leaf size. Microphylls would allow for more light to penetrate the canopy since they project only a short distance from the stem, even if many are found on a given stem. Megaphylls of ferns or seed plants would, in contrast, capture most light, making for a much dimmer understory. As a result, one would expect much larger and more vigorous understory plants in the forest with microphylls due to the much greater amount of solar energy available to understory plants in that case.
Work Step by Step
Look at pictures of microphylls and megaphylls in the textbook and think about how much more light is captured by the latter. Extrapolate from one leaf or group of such leaves to a whole forest, and then think about forests you may have walked through— they had megaphylls and were dim under the canopy of leaves.