In love with a Rare mare

Starlight did say that nothing can simply "disappear", implying that magic cannot simply cause mass to cease existing. This implies that the mass has to go somewhere. What it is turned into is then up for debate. When MeanLucario says "a lighter element" I presume that they mean some type of gas. It is possible that the excess mass is converted into something like hydrogen, helium, oxygen, or nitrogen (or a combination thereof) which would simply become part of the atmosphere. This would probably cause problems in our world, but in Equestria, where pegasi are in charge of controlling the weather, they may have a process by which they can balance out the air elements. (This would also explain the high difficulty of a transmutation spell, as the unicorn in question not only has to change the object's shape, size, mass, etc., but also be able to convert the excess mass into lighter gasses in an instant.)Furthermore, if my above theory is accurate, what Trixie did in the "Teacup Scene" is extremely dangerous, as converting so much excess mass at once could drastically change the air pressure of the room, which could cause difficulty in a pony being able to breathe. (The pressurized air may force itself into the ponies' lungs, or vice-versa, where the air would not be able to be exhaled, due to the pressures.)