They have the same basic anatomy, but the difference between big and small breeds of dogs extends from appearance and behavior even to interior functions, such as heart rate.
Smaller dogs -- those up to about 30 pounds -- tend to have higher heart rates than larger dogs. Generally, breeds with normal weights over 30 pounds at maturity have normal heart rates of about 60 to 120 beats per minute. Smaller dogs, on the other hand, typically have heart rates of 100 to 160 beats per minute. Your dog's exact heart rate depends not only on his size but also on age, health and activity level. A heart rate between 60 and 160 beats per minute is considered normal for a dog of any size; anything outside that range is cause for alarm. Contact your veterinarian if you suspect a problem with your dog's heart rate.