Epoxy curing is exothermic. A thick mass traps heat, which speeds the reaction further and can cause cracking, yellowing, smoke or warped molds. Coating epoxies: about ⅛–¼" per pour. Deep-pour formulas: often 1–2" or more, but read your label. The volume matters too: narrow, enclosed molds hold heat more than wide, open ones.
Pour the next layer once the previous one is firm but still slightly tacky (the "green" stage) for the best chemical bond; otherwise sand it lightly first.