To expand my previous comment:
The animals which entered the Ark were a representation of their particular species, for example, lions, tigers, bobcats, lynx, cougars, leopards etc are all big cats and share a common ancestor. In the same way bears come in all shapes and sizes - grizzly, panda, brown, koala, polar etc but all share the same origin. All breeds of domesticated dog descend from wolves, including chihuahuas and pugs.
The Bible states that God told Noah to:
"Bring into the ark two of all living creatures, male and female, to keep them alive with you. Two of every kind of bird, of every kind of animal and of every kind of creature that moves along the ground will come to you to be kept alive".
So Noah wouldn't have only needed to include a representative of each family of animal. Fish and other sea creatures were not included as they could survive in the water.
Some people also think they would have been juveniles. In this case the animals would take up far less room, be less territorial, less inclined to kill other animals, wouldn't try to breed and would not need so much food.
Research Biologist Dr. Nathaniel Jeanson writes:
'Only about 30% of all mammal families that ever existed are alive today. Assuming a similar percentage for amphibians, reptiles, and birds, we can estimate how many total families in these groups of creatures ever existed. If 510 families exist in these groups today, and if this represents just 30% of all that ever lived, then a total of 1,700 mammalian, amphibian, reptilian, and avian families once existed on earth. This number—1,700—represents an upper estimate of the total number of kinds that Noah took on board the Ark.'
The Ark is estimated to have been 550 feet long, 91.7 feet wide and 55 feet high, with 3 levels and multiple rooms within.