Potato and Macaroni Salads soak up all the mayo when I store them When I make either potato or macaroni salad, I always have a problem the next day with the salad soaking up the mayo so it looks like there was no mayo added.  I DO feel that I add a good amount of mayo.  

I don't add a lot of ingredients, just a little salt and black pepper, chopped onion and celery and maybe a little parsley, never any eggs or mustard.  I don't know what I'm doing wrong because I always let the potatoes cool before making the salad, and I always rinse my pasta in cold water after cooking and let drain for a while before mixing.  

A deli near me makes potato salad and I'll have it for days and it will still look like it was just mixed with mayo; the potatoes in their salad (macaroni too) never absorb the mayo.  Does anyone have a remedy to this problem?
It is highly likely that they simply add enough mayonnaise and other ingredients of their sauce that even after the macaroni or potatoes absorb their maximum amount, there is enough left.