If you are doing any sort of exercise regimen and you are devoting equal amounts of time to working out your lower body and upper body, you must have noticed that its much easier to see more strength increase, as measured by the amount of weight you can lift and how many reps you can perform, in your lower body than upper body.
In a typical 12 weeks of progressive loading based routine with exercises done with proper form and performing sets to failure, its not unusual to see your strength or total work capacity increase by 25% for the lower body compared to 10-15% for the upper body.
This is because of the much higher muscle density in our lower bodies for both men and women as out lower bodies support the entire weight of our bodies so we are just naturally built this way.
What that also means is that you can typically do a 2:1 split for upper to lower body exercise routine. If you are working out 3 days a week, Day 1 can be a upper body routine based around Chest and Arms, Day 2 can be lower body routine based around Legs, and Day 3 can be upper body routine again based around Shoulders and Back.
Its one of the simple rules to keep in mind when embarking on any exercise regimen for a proportional body development.