Quantity.
The original usage of Gourmand was synonymous with gluttony and excess; while a gourmet might be satisfied with exquisitely prepared micro portions tucked away within an expansive plate criss crossed by a drizzle of ??, a gourmand wants the full stack pyramided to the maximal stope angle.
2: one who is heartily interested in good food and drink
https://www.merriam-webster.com/dictionary/gourmand
Been a long time since it was purely about quantity.
> The original usage of Gourmand was synonymous with gluttony and excess; while a gourmet might be satisfied with exquisitely prepared micro portions
Even if you removed the word "might", they wouldn't be opposites. With it, they're even further from opposites.