They were mostly loaned as such (from German and French for the most part), if they aren't familiar or simply popular folk forms (like Yegór <<< Gr. Georgios), in which case shortening becomes generally expected. On the other hand, some popular Russian Orthodox folk names got -ей/-ий through hypercorrection; e.g. Kliméntiy <<< Lat. Clemens (cf. Ch. Sl. Klimént), Kondrátiy < Gr. Kodratos < Lat. Quadratus (Ch. Sl. Kondrát).The short names like Антон have originated non-phonetically, being either borrowed in the shorter form or randomly restructured.