Why is there liquid in my macaroni salad, potato salad and tuna salad the day after making them? When I make potato salad, macaroni salad, tuna salad, and sometimes ham salad, it seems like there's always a puddle of excess liquid at the bottom of the storage container the next day.

Does anyone know what is causing this or how to prevent it from happening? 

I always cook the pasta in water with oil to limit absorption, and I always try to drain it thoroughly. I also always use regular mayonnaise or salad dressing instead of the light ones which have more water in them. A day later, I am always draining off the excess liquid and remixing with mayonnaise all over again. 

The supermarket salads never seem to have this problem: what are they doing differently? Can anyone help???
Have you tried baking pasta, after boiling it in the water? 

I highly recommend you bake the pasta in the oven on very low heat, for 20 minutes, then you will see that there is no more liquid. 

Also it gives the macaroni a spongy condition which makes it able to keep the liquid in it. 

I believe if you bake the pasta before mixing with mayonnaise, you will get a good result.