increase, bear children
孳 centers on growth through reproduction: increasing in number by bearing children or multiplying. The idea of increase is tied to the natural result of giving birth.
孳 combines 兹 (likely phonetic, suggesting increase) with 子 (child), expressing the idea of multiplying through offspring.
The top 兹 looks like a pair of growing sprouts, and 子 is a child. Picture sprouts multiplying and children being born, both showing increase.
For ジ, imagine a gene (ジ) that causes rapid increase: gene -> ジ, and the gene makes children multiply.