Black insect with antennas and large abdomen
What species is this insect from and what is the insect called?


This is a Meloe proscarabaeus, most commonly known as a black oil beetle. Looking at the antenna of the insect in your provided image, it's clearly a female. 

M. proscarabaeus


Males have a "kink" in their antennas, females don't.
    


  What caused it to have such a large abdomen?


The primary reason/cause for the large abdomen is eating. Initially, the abdomen is much smaller, but after eating it will rapidly swell. Another cause for such an abdomen is pregnancy.