Frank - the two aren’t unrelated. Obviously you feel the weight in your mouth, i.e. mouthfeel. And like all descriptions of wine, there isn’t an agreed-upon standard because we all experience things subjectively.
But generally mouthfeel would literally have to do with the way the wine feels and the way it feels is dependent on any number of factors. Tannins are one factor and how ripe or mature they are is another. So if you have a young wine with big tannins, they bind with proteins in your mouth and have some astringency as a result so your mouth can feel scrubbed out almost. Combine those with some harsh acidity and you have a young, old-school Nebbiolo that will be tough to take. If those tannins are from stems and seeds and are slightly green, it will be all the more rough feeling.
But say you have something 30 years old from a great vintage and the winemaker handled the juice carefully. Those tannins will be very different, many will have dropped out, and the wine won’t feel like it has a texture, it will be silky and smooth.
IF you take a white like a Chardonnay and leave it on the lees for a while, stirring occasionally, and let it go through malolactic fermentation, the acidity will be transformed from the harsher malic acid to the easier-to-take lactic acid, and the lees will add some weight and maybe some bready flavors, and that wine will feel different than another one from the same vineyard that was not treated the same way. The wine might seem “creamy” whereas the sister wine that didn’t go thru the malolactic fermentation or lees stirring will seem less creamy and malic acid is a tougher acid in the mouth, so the experience will be completely different.
In addition to the feel however, the extra particles from the lees or in the case of the red, from the tannins, will add to the weight of the wine. Particulate matter makes a wine feel heavier in the mouth - a young fresh Rioja that will be a Gran Reserva will generally seem heavier than it’s older brother that has been laying down for 40 years.
Sugar adds weight as well, which is why some of the sweet wines from Australia are so thick and heavy, but even a bit of RS can often make a wine seem heavier than it would w/out the RS.
But all that is subjective. Pinot Noir has a thinner skin than something like Petite Sirah, so generally will be less tannic and one might say would be lighter in body. But if you pick really ripe and it’s fermented whole-cluster and punched down a lot and put into young oak, it can pick up tannins from the skins and stems and seeds and wood and therefore seem heavier than a wine from the same vineyard that was prepared differently.
Then there’s the question of the individual drinker’s experience. If you’ve tasted thousands of wines from all over, you may well have tasted some really heavy wines and some really light ones. Someone who’s tasted fewer might have a completely different frame of reference. So in the end you can compare wines to each other as being silkier or rougher or heavier or lighter, but that’s about it. There’s not some agreed-upon objective standard that we all subscribe to so something you consider light-bodied may be something another person considers medium-bodied.
As an aside, I would say rather than read about what a wine or grape is supposed to be like, do exactly what you’re doing and taste for yourself. Seems like you completely got it in your tasting and the confusion is only from what you read or heard from someone else.
Cheers.
