Deep sea creatures often evolve into giants due to a phenomenon called deep-sea gigantism. There are several theories, though the exact reasons are not entirely understood. Here are a few factors that might contribute:

Adaptation to Extreme Environment: Deep-sea environments have high pressure, low temperatures, and limited food resources. Larger size can help organisms withstand these harsh conditions.

Reduced Predation: In the deep sea, there might be fewer predators compared to shallow waters. Being larger could deter predators or make it easier to defend against them.

Efficiency in Energy Consumption: Larger animals might be more energy-efficient in utilizing scarce resources in the deep sea. A larger body could store more energy or require less energy relative to their size compared to smaller organisms.

Stability and Mobility: Larger size might provide stability and aid mobility in a habitat where movement can be challenging due to the darkness and high pressure. Larger bodies might help them navigate more effectively.

Longevity and Reproduction: Some theories suggest that larger size could contribute to longevity and reproductive success in these extreme environments.

However, not all deep-sea creatures are giants; there's a wide variety of sizes. Evolution in the deep sea is still an area of active research, and multiple factors might contribute to the variations observed in deep-sea organisms.