Question: If you are heavier you will be stronger wether this is by fat or by muscle I think we can all agree on that. (I'm not talking about overly fat because at that point it would probably not hold true due to health reasons)

So if you have excessive fat, will you gain more muscle because you can lift heavier then the version of yourself who has the same amount of muscle but a lot less fat? Or am I missing something here?

Scenario:

Say there is a version of me of 70kg with 10% body fat and a version of me of 80kg which has the same amount of muscle mass but higher fat percentage. 

Both training the same exercises but lifting the most I can lift in a secure way, will the fat version gain more muscle mass?

Answer: 
Most sports require lean performance as fat adds nothing but sport baggage. Good 40 yard times and slam dunks require it. IF you push your fat percentage below 5% other organs including the immune system will be compromised. Many Olympic performances have not gone as expected for this reason. Sumo wrestlers, cannon ball targets and cold water swimmers are special needs events and are associated with specific health related compromises. For the average human, gravity is a daily reality that fat as baggage creates passive exercise, still low body weight is associated with longer life expectancy and generally greater health. Dietary fat generally concentrates man made contaminates, cholesterol, and has lower satisfaction of hunger per gram eaten.
Elite athletes often increase the duration of peak performance by managing fat intake.
Muscles make you stronger, not fat, fat will make you slow and lazy because Muscle is more metabolically active than fat. Since muscle is denser, if you compare two equal sizes of fat and muscle, the muscle would weigh more. The density of fat is .9g/ml whereas the density of muscle is 1.1g/ml. these numbers can vary depending on numerous factors such as race, sex, or body type.

High body fat percentages are associated with raised risk for obesity-related diseases such as type 2 diabetes, heart disease, high blood pressure, and even certain cancers. The average everyday American woman should look to be somewhere between eighteen and thirty percent body fat, whereas men should look to be between ten and twenty-five percent.

Apart from fat and muscles other things also matter.

1. **smoking:** if someone smokes then he gets tired first in stamina based exercises.
2. **proper eating:** the one who eats properly and takes a balanced diet can generate more energy and have more stamina.
3. **Injury:** one cannot perform well if he is injured.
4. **diseases**: if one is suffering from any major or minor diseases then his body will not function properly and he will consider weaker.
5. **Bone density**: just consider someone is having stronger bone, obviously he will be more solid and stronger form inside. stronger bones means stronger base.