The more of a substance you have the longer the reaction will take. If you have a big pot of water and a little pot of water. The little pot will heat up quicker then the bigger pot because it does not need to fill a space up. Think of it as diffusion. When you have a 500 and 50 mL beaker, and you add a blue and red dye to them both. It will only take about a minute for the dye to almost completely mix together in the 50 mL. While the 500 mL it will take much longer for dye to mix all together then it did in the 50 mL. When it is a bigger substance it takes longer for a reaction to fully expose the substance to that reaction. If you have food, if you only have one piece of food you only need to put it in for 30 seconds to 1 minute. While if you have multiple pieces of food you will need longer time to be able to heat all of the pieces. If you have a small bowl of soup and you put it in for a minute the middle will be warm. But if you had a bigger bowl and still put it in for only a minute then the middle will be cold because you did not give it enough reaction time to heat the whole substance.
