Here is a comparison of the ingredients (UK):
Hellman's Light - 264 cals per 100g
Water, rapeseed oil (25%), spirit vinegar, modified corn starch, sugar, salt, free range EGG yolk (1.5%), cream powder (MILK), citrus fibre, thickeners (guar gum, xanthan gum), MUSTARD flour, lemon juice concentrate, antioxidant (calcium disodium EDTA), natural MUSTARD flavouring, paprika extract, sunflower oil.
Hellman's mayo -726 cals per 100g
Rapeseed oil (78%), water, pasteurised free range EGG & EGG yolk (7.9%), spirit vinegar, salt, sugar, sunflower oil, lemon juice concentrate, antioxidant (calcium disodium EDTA), flavourings, paprika extract
Yes, water is added and there is less oil - the main ingredient that compensates for the lack of 'emulsion' due to the extra water/less oil is xanthan gum. I use it to thicken sauces and it has virtually no calories. It thickens a sauce without adding (unwanted) flavour or calories. Its a very useful product. I thing that guar gum is similar but I've never used that.