Glycine in magnesium glycinate
If there are no other ingredients like citric acid- 1800mg. It is probably magnesium BISglycinate. For each Mg ion, there are two glycine molecules.

After a calculation, if it's MagBisglycinate, for 200mg Mg you should have around 1235.5mg glycine. It probably means there are other ingredients or simply glycine that left unreacted. If it's simple Magnesium glycinate than you should have 617.7mg glycine for 200mg Mg.

Correct me if I'm wrong.