It is not accurate to generalize that male spiders are always smaller than female spiders. While many species do exhibit sexual size dimorphism, where one sex is noticeably larger than the other, the specific pattern can vary greatly across spider species. In some cases, male spiders may be larger than females, while in others, the size difference may be subtle or nonexistent.

Here are a few reasons why sexual size dimorphism occurs in spiders:

1. Sexual Selection: In many spider species, males compete for access to females. Larger male spiders may have an advantage in these competitions, as they may be more successful in attracting females or defending their territory. This evolutionary pressure can lead to the selection of larger males over time.

2. Mating Strategies: The size difference between male and female spiders may also be related to mating strategies. In some species, male spiders use their webs to capture females. Larger male spiders may be able to construct larger and stronger webs, which can increase their chances of catching females.

3. Resource Acquisition: In some spider species, females may require more resources for reproduction, such as larger bodies or more eggs. This can lead to the evolution of larger females in order to support the demands of reproduction.

4. Variation in Species: It's important to note that sexual size dimorphism is not a universal trait among spiders. Different species may have different patterns of sexual size variation. Some species may have relatively similar-sized males and females, while others may exhibit pronounced differences.

Therefore, the idea that male spiders are always small while females are giant is not a generalized rule and varies greatly among different spider species.