Converting Video Problem

[123Justin]

hey everyone.. so i have this wierd problem, here goes... i am trying to make room, so i converted some of my videos, i made the videos lower resolution but same quility, so just by lowerng the reslution i alreayd took a huge chunk of MB out of that file... so den after i did that, i convert the files again.. this time with everything the same, quality ..resolution.. everything, but when the files r done converting, it came out to be smaller in size... i find that wierd cuz if its the exact same quality as the one before i converted.. how is it smaller... so i kept doing this, i stopped after the 3rd time, and evreytime the files would drop in size, not by alot tho, maybe around 10-15mb each time, the orignal file is 80 mb. plz help. thanks