Having lived in France for two years now, the people I live with seem to prefer "le repas de midi" and "le repas de soir" instead of "déjeuner" and "dîner". However, I got this wrong because in the US (my region) I grew up distinguishing meals by their size. Here our principal meal is (circa) midi and it feels strange calling it lunch (in English). Oh, well, vive la différence!
"Le repas de midi" and "le repas du soir" are descriptors rather than the meals' names, and they have 5 syllables to say the same thing as "le déjeuner" and "le dîner".
When it comes to the size of our meals, there can be huge differences from one person to another. Depending on your activities, whether you work or not, can have lunch at home or not, have a company restaurant or not, have deep pockets or not, etc. your lunch can be very light or the more substantial meal in your day.
The French are not very good at having proper breakfast, so some are really too hungry at lunchtime to only grab a snack on the go. Others still like to gather with family at dinner time and have a proper meal, meaningful conversations, a chance to check on the children's manners, etc.
No, there is a clear difference.
"This is lunch" shows a thing and describes it as "lunch": this = lunch (sandwich, pizza...)
"This is for lunch" shows a thing and describes it as intended for lunch: this = to be used/eaten/drunk at lunch (fork, sausage, bottle of water...)