'Usually when there's a disparity the serving size by volume is usually larger than the serving size by weight but this time it was the other way around to where the serving size by weight was larger. So it made me wary that maybe I was eating more calories than I thought I would if I went with the weight.'
I agree that the weight is more accurate. In this case, it seems to me that you wouldn't be eating more calories with 3 tablespoons. You'd be eating less. You'd be eating less calories than you thought you'd be getting in the tablespoons.
My reasoning is: the weight is more accurate. So, you say, 'the serving size on it says 30 grams or 3 tablespoons. When I measure out 30 grams it comes out to more than 3 tablespoons though.' Let's say 30 grams is 50 calories (for example). But you measure it out to be three tablespoons. That is going to be less than what the actual weight is. To meet the actual weight, you'd have more than three tablespoons. So, you are going to be eating less than the actual weight with YOUR measure of three tablespoons and that means less than the actual measured calories for 30 grams. So, you eat less calories - probably less. But definitely not more.
To put it another way: when you have to worry is when the weight tells you the food is x number of calories but you need MORE of the food to fill 3 tablespoons. That's when you fill those tablespoons with more calories without realizing it.
That's the OTHER case. Weight and the number of calories associated with that weight of food doesn't change. It's constant (given that calories are an estimation for most foods, anyway). So, you can't eat more calories using weight. You can only eat the number of calories associated with the weight. It's measuring by tablespoons that is dodgy.
It's a bit confusing. Weight is more accurate.