When an aqueous extract of a substance is said to be administered at a certain quantity, is the quoted mass analogous to one as original substance?
You probably get the $\mathrm{LD_{50}} = \pu{1585 mg/kg}$ value by reading the abstract of the paper. That's why you got confused by that value. However, it is not $\pu{1585 mg}$ of fresh leaves. It is actually $\pu{1585 mg}$ of dry leave extract. I mean dry residue obtained by removal of water from an aqueous extract of dried leaves. The authors have made a stock solution of that dry residue in  $\pu{200 mg/mL}$ concentration by dissolving in enough distilled water ($\mathrm{pH} = 6.7$). Thus, $\pu{7.9 mL}$ of that solution is lethal to a rodent weighing $\pu{1.00 kg}$. On the other hand, since they have used rodents averaging $\pu{100 g}$ of body weight, each of them need only $\pu{0.79 mL}$ of that solution to be lethal.
Hope this explain what you are looking for.

Reference:
Olufunsho Awodele, Ibrahim Adekunle Oreagba, Saidi Odoma, Jaime A. Teixeira da Silva, Vincent Oluseye Osunkalu, &quot;Toxicological evaluation of the aqueous leaf extract of Moringa oleifera Lam.(Moringaceae),&quot; Journal of Ethnopharmacology 2012, 139(2), 330–336 (DOI: https://doi.org/10.1016/j.jep.2011.10.008).