Why do you have weight restrictions?... The manufacturers of parachute equipment and the Federal Aviation Administration have set stringent requirements on the operating limits under which parachutes are designed to work. Specific testing of how parachutes are tested had how much weight they can safely support and at what speeds they are designed to open are detailed in FAA Technical Service Order (TSO) 23B, C & D. To make a very long story short a parachute system is designed for a maximum exit weight (the weight of the parachute, jumpers, and gear as they exit the plane).
Let's make an example of a tandem; the parachute itself weighs about 65 pounds, your tandem instructor might weigh 210 pounds leaving room for a student to weigh up to 225 pounds. "WAIT A MINUTE, I thought it was about 210 pounds was the limit?" Ok, you're right, let me explain more...
If we have an instructor who weighs less can they can in theory take a larger student, but, the instructor must be physically capable of controlling the student in the airplane, in freefall, under canopy and landing. We reach a point where the smaller instructor just can't safely manage the larger student.
OK, but in your example above you could take a 225 pound student. Yes, in theory that is correct, but, that is under optimal conditions.
Alright, what are those conditions? Weather, elevation above sea level where the parachute center is. Weather - On a cool day with weather in the mid 70's and a steady breeze blowing from an open and clear direction at a parachute center which is at sea level, sure, no-problem with taking a 225# student. But as air gets warmer and the higher the humidity the higher the density altitude.
Density altitude is a very straight forward term that places a value on how dense the atmosphere is. Just to use round numbers and not get too technical, if we were at an airport which was at sea level on a 95 degree day and the humidity in the 80's the density altitude might be something like 2,500 feet. What that means is that something that flies, airplanes, balloons, and yes, parachutes fly like they were actually at 2500 feet above sea lever. They are just less efficient at higher altitudes and don't work as well.
What does all that mean. Exit weight 500 pounds, 70 degrees, 20% humidity, steady 12mph winds to land into, landing area at sea level, NO PROBLEM, I would expect to be able to give you a very soft stand up landing. BUT, Exit weight 500 pounds, 90 degrees, 85% humidity, no wind, at an airport say 1500 feet above sea level, NO WAY, the possibility of a harder landing just isn't worth it.
Also each major component of a parachute has a placard on it stating it maximum suspended weight and the maximum speed the parachute components are designed to withstand.