Why cant I exert more strength or energy?

I am the fittest male in all my classes..college student standing 5'7 at the age of 19 and weigh 65kg. I sort of have visible six-packs (They pop out during a good exercise) and a pretty toned body. I dont look bulky, buff or scrawny but I do look athletic. I have visible muscles all over..I guess I lack having much fat around the legs..arms..waist..shoulders. I drink my daily milk..water..healthy diet everytime and never miss out on my cardio exercises. At school..whenever I end up in an arm wrestle match..or sumo wrestling..sprint racing or high vertical leap jumping, I always end up losing or never become one of the first few champions of any physical event. I feel like my muscles..pecs..arms are inflated with AIR!! My potential energy or strength or results of my healthy body never SHOWS. Its like I am as strong as some weak skinny person. I can beat most of my classmates who are about my size but those who are a few inches taller can nail me almost in anything..even at jumping or pushing. How can I exert MORE strength or ENERGY? Please help me out..I have a GREAT stamina.. im a long-distance runner..with good endurance.