Volume vs Weight - Measuring Ingredients

I sometimes skip weighing my ingredients, which maybe I should stop, but am very curious about the some of the pre-defined measurements on cronometer.

For example, a large, raw onion on cronometer comes in at 150g. I just weighed an onion I have in my cupboard, and it weighed 350g. And I don't even consider it a "large" onion, I consider it a "medium" size onion from all the ones in the supermarket. Are the onions in California supermarkets abnormally large? My onion has about a 3.5 inch diameter.

The same with bell peppers. On cronometer, a large, raw green bell pepper comes in at 164g I just weighed what I consider to be a medium sized green bell pepper and that weighed 250g. It was about 3.75 inches in diameter and 3 inches tall. On cronometer, the measurements for a large, green bell pepper are 3 inch diameter and 3.75 inches tall, giving it a volume of 26.51 inches^3. My bell pepper has a volume of 33.13 inches^3. Even if i were to use the gram to volume ratio for cronometer's stated large bell pepper to get the weight of my bell pepper based on volume, it would come out to be 201g, which is a 20% error.

I've been using the volume measurements for awhile now when creating recipes, like I added two large bell peppers, but based on weight I should have been putting three large bell peppers. It just makes for a large discrepancy when it comes to my own actual reporting.

The solution to all this I guess is to create my own custom recipes for these single ingredients and then create my own serving sizes of small, medium, and large onions/bell peppers if I do not want to weigh the ingredients every single time.

Originally, I was going to post this in "Ask An Expert", but I'm not longer sure what my question is. Perhaps it is simply, why are the volume measurements for small, medium, and large so off for what I consider to be small, medium, and large onions and bell peppers? If this is the case for me, is it the same or different for everyone else?

Likewise, if it is the same for everyone else, then are the pre-defined measurements for small, medium, and large onions, bell peppers, and whatever else fruit & vegetables, causing more harm than good? Harm, in this case, being defined as inaccurate reporting, given the whole point of cronometer is to get an accurate picture of our daily caloric/nutrient intake.