why is it the little dogs that are the most aggressive seeming often times?

because the "littlest dogs" have not had their genetics, their original temperaments altered to the degree of the "largest" dogs....wolfhounds, akitas, giant and standard schnauzers, dobermans, and the rest of formerly assertive breeds have had a century of years of breedings intentionally planned to eliminate their original natures promoting dominance and assertiveness.

obviously small dogs usually cannot inflict mortal injury as the giant schnauzer of old was capable of inflicting on intruder or visitor, thus pressure on chi breed fanciers as example (law suits, reputation etc) was not as great and kids soon learned not to tease without (hopefully) major damage.

temperament is an inherited trait, through selectively breeding dogs with bite inhibition mild natures are encouraged.

Cause they like it when you kick them like a football

I just say ditto to Jojo and that would be my answer too. 

and to anon who thinks they're not and that's an internet prejudice,  try walking a big dog for 30 yrs and see if you say the same thing and it shows even more when you live with both.   My JR cross died of a heart attack when lunging himself into my Rottweiler neck !    Most of us with multi breeds will say the litte dog is the boss. In this picture my little terrier is most definitely saying....  "If you find anything at all it is mine"   and it would be.  

Just like human beings, they can have a little man syndrome.

because depend on the breeds  .back old days hundred years ago dogs use to was aggressive toward of strangers and other pets and some time including children .if the dog was not raised with family who has other animals and children before. some dogs are used to train to kill and hunt for sports and family companion , for working breeds. and some used to was trained to fight against fighting sports means put two aggressive dogs in a place and bad people who train their dogs to kill and fight with other people dogs for money and when they have a injury dog from the dogs fight usually they abondan their dog on the streets or drop them at the animals pounds  .they use to do those bad sports doesnt care about their pets at all. so its llegal now in U.S that there no allow having pits american and american bull terriers pure breeds and mix breed most apartments and community some places does allow having those breeds dogs .but most has banned those kind of breeds. so that why most dogs become so aggressive when they end up for adopt .that why it better ask the animals pounds and rescuers before you plan to buy a dog .if the dog are aggressive toward of other people and get along with younger children , other animals such as cats, dogs, birds, ect.  you can train the dog not to be mean all the time. 

Because people pick & tease them.  Thinking it is cute to see a little vicious dog.  Picking on them & teasing them brings out their aggression.  If the little dogs were bigger dogs, picking on them would end up in a bite & with bigger teeth that would hurt.  Little teeth verses big teeth.  Owners fault for allowing that kind of treatment.

Mostly they put on a big show of barking and growling, especially towards larger  dog because they fear the larger dog and think they can scare it away with a show of aggression.

However....Some small terrier type dogs are naturally belligerent, and will try and prove their status by attacking other dogs quite viciously for no reason.  

The aggressive ones act tough until you make a move toward them. Then they retreat.

Maybe it's their defensive mechanism to deter an attack on the wee little shits

Because when it comes to humping your leg............they NEVER get to finish.