It's because, as established in the first chapter, one's mana in this setting is almost purely based on their genetics. Hence magic maniac Loid being utterly terrible at it in his former life. There probably is some amount of usage boosting too, but if that's, for example, capable of doubling the base amount, a base of 1 still only goes up to 2, whereas the clone should be starting with at least similar power to Loid though knowing the series so far it seems unlikely he got more than a quarter or so.