Oh, then it would be "un déjeuner léger".
So for your sentences :
"J'ai encore faim parce que j'ai seulement pris un déjeuner léger."
"Je mange un déjeuner léger pour garder mon appétit."
Note that "léger" can switch position sometimes, even if I could not say if it's official.
"Un repas léger" or "un léger repas" are both used for example. If we have a look at French rules, I guess it would be "Un repas léger", but as there are many exceptions in French, can't say that for sure.
This is very weird for me. I know english and spanish and its funny to me that the construction of the words breakfast or desayuno, are the same for both languages. When we eat during the day our meals are about five to seven hours apart. Typically we eat say at seven, tweleve, and then around seven. So from dinner to breakfast we are not eating for almost twelve hours, which is considered a fast, abstinence from eating. So in english the word breakfast literally means break the fast, stopping not eating. Same in spanish, fast means ayuno, des is like stop, stop the fast.
However, I don't get the french construction. Jeuner means fast, so dejeuner should mean break the fast, but dejeuner, means lunch not breakfast?
Indeed, "déjeuner" means "lunch" and "petit-déjeuner" means "breakfast".
Languages evolve differently. Maybe once in the past, "breakfast" and "déjeuner" had the same meaning.
But if it can make it less weird, "petit-déjeuner" is only adding "little", so it's still "breaking fast".
Interesting. I was also wondering and since from above I see your French, I can ask, maybe the reason its called petit-dejeuner, is because the French as a custom eat very small breakfastes, like coffee and toast and that's it and really break the fast with a nice big lunch. Is that the custom? If not maybe it was back in the day.
Hmm. I don't recall any specific custom like this, and I myself always eat quite a lot on the morning.
I found this, it's in French, but maybe you'll be able to understand anyway :
It's basically saying that before, the original names were :
"déjeuner" for "breakfast", "dîner" for "lunch" and "souper" for "dinner."
But in Paris, people kept eating lunch later and later, probably because of staying late at night while partying, and thus all the cycle went down a notch to make "souper" disappear (only old people use it in France nowadays).
Then the names for "breakfast" and "lunch kept changing, to finally end up with "petit-déjeuner" and "déjeuner", respectively.
It also says that "souper" is still used in other countries speaking French (Switzerland, Quebec, etc...)
So petit-déjeuner means breakfast. So If I want to say my small breakfast I can say "Mon petit-déjeuner léger". Léger meaning light? Also for my boyfriend you say "Mon petit ami" or my girlfriend "Ma petite aime". So how would I say my small boyfriend or my small girlfriend?
I would say "Mon léger petit-déjeuner" personally, but both could work.
"léger" can mean "light", but it has several meanings, just as "light" doesn't always mean "lumière" either.
For "my small boyfriend" or "my small girlfriend", you could say something like "mon/ma copin/copine de petite taille".