If something says low fat (not JUST low fat, but low calories/salt/sugar as well), would you not say it is healthier than something that is full fat? No, I'm not one of those people who pick up items from the supermarket that only say 'low fat', I make sure I look for low calorie/sugar/salt content as well.
No, fat isn't bad for you, you die without it, it's vital for thousands of processes in the body. Fat fills you up significantly more(per calorie) than carbs do, it filters your insulin response(well in particular the omega's do) and in general "low" fat food will have a higher portion of saturated fats.
Eating something with 20 grams of fat that is 5grams saturated fat, 10 grams polysaturated fat and 5 grams monosaturated fat is SIGNIFICANTLY healthier for you than eating some low fat meal that has 10 grams of fat removed but leaves you with 5 grams saturated fat, 4 grams poly and 1 gram mono.
It's to a degree(not via the same processes, but in terms of thinking about it) cholesterol, there is good and bad and a meal with a BALANCE of both is good for you while a meal with less cholesterol but all or mostly bad is very bad for you.
In general what low fat meals do is take all the constituant parts rather than an existing product. At which point they throw in cheap refined carbs, cheap refined sugar, cheap horrible bad fats and you get your statistically great low calorie low fat low salt meal... but there is also nothing good in it.
It's hard to make a say macaroni cheese with good quality everything then "remove" the bits you don't want. So mostly these meals are made up of individual bits. As with most supermarkets they generally use cheap crap. There is a reason in america that corn syrup is in everything, because it's stupid cheap and they can fill up products with it for cheaper than better quality less refined for you foods.
Calories aren't king for losing weight, or controlling it. People seem to forget your body actually uses what you put in it for more than just energy, more energy is spent in the day maintaining your body. Feed your body bad fuel and bad parts and your body can't work efficiently or do a whole lot with it.
Tefal, fullness isn't about volume and it's also self defeating. if you take out fat or other things and eat more of other things, you just get used to eating a larger volume.
Fullness isn't due to volume for the most part purely because the stomach is INSANELY stretchy. sometimes you can eat an entire pizza, sometimes a couple scrambled eggs leave you so full you couldn't eat anything else... there is a reason for that. Anyway it's self defeating because on any diet EVERYONE slips, if you are used to eating a large volume, when you slip... you're simply more likely to eat a large volume of crappy food. It's a mental thing, a learned behaviour. When dieting you should be eating smaller portions and training yourself to only want to eat smaller portions.
Because the stomach is "adjustable" in size, significantly, fullness isn't based on volume. Almost anyone who eats higher fat/protein/low carb meals will feel fuller on less calories and be less inclined to over eat.