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Understanding the metabolism of poikilotherms—organisms like reptiles, amphibians, and some fishes—reveals a fascinating interplay with their external environment. Imagine their bodies like engines that rev up or slow down depending on the ambient temperature. When things heat up, their metabolic rate accelerates; they become more active, digest food faster, grow more quickly, and reproduce more efficiently. In contrast, a chilly environment cools down these processes to a sluggish pace, leading to inactivity or even hibernation.

Poikilotherms do not have the luxury of internal temperature regulation like mammals or birds. Instead, they must find suitable external conditions to thrive. The right temperature, which is typically found in warmer climates, is like hitting the sweet spot for their bodily functions to operate optimally. Hence, tropical locales, acting like a natural thermostat set on the 'ideal' setting, create the perfect stage for these creatures to live out their best lives.

Delving into the broader effects of temperature on organisms, it's not just about speedier metabolisms or lazier days; it's a matter of survival and ecological fitness. Temperature affects everything from an organism's cellular processes to its behavior and life cycle. The impact can be so profound that it often dictates where a species can or cannot live.

For example, a rise in temperature might cue a burst of growth or herald the onset of mating rituals. On the flip side, a drop in temperature can signal a time to conserve energy, even to the point of entering a state of dormancy. These temperature-driven life processes ensure that species are synchronized with their environment, taking advantage of the best conditions and avoiding the perils of the worst.

In terms of size, think of tropical and subtropical ecosystems as giant playgrounds for poikilotherms, where not only the climate but also the rich biodiversity and food availability come into play. These environmental factors are like a buffet that's always open, providing not only the right temperature but also ample sustenance for growth. These favorable conditions alleviate the energy struggles faced by poikilotherms in less congenial habitats, where finding enough food or simply surviving the temperature fluctuations can be a daily challenge.

Big sizes come with big advantages too; larger poikilotherms can dominate food sources, fend off predators more effectively, and optimize their reproductive strategies. It's no wonder that, given these optimal conditions, poikilotherms in tropical regions tend to grow larger and more plentiful compared to their counterparts in more temperate zones.