The word is out that toy breeds are harder to train...
Pookie, the four-pound longhaired Chihuahua CAUGHT peeing on the carpet! Pookie squatted and peed just a few drops. In a matter of seconds the pee soaked into the carpet. Had no one been looking at that exact moment the incident would have gone unnoticed.
Some factors are:
Many breeders raise toy breeds in a big 4' x 4' whelping box, and they do not learn at a young age that there is a place to eat, sleep and a separate place to eliminate. When purchasing a puppy, ideally find one that hasn't learned bad habits that you will have to correct. Breeders of larger breeds tend to put them in a big room or outside—large dog waste cannot be controlled in a kitchen whelping box after six weeks as it can for toy breeds. There is just too much, and the large breed litters are larger in size as well.
Another factor that I have learned by my own error is that many people, because toy breeds are so small, pick them up (every hour or two), carry them to the door and carry them outside to the designated spot. You should make them walk on their own, as a big dog does. Constantly carrying them doesn't make them go to the area on their own. They need to walk to the door, walk out the door and walk to the designated area on their own as puppies, exactly as expected as an adult. Remember to always use the same door, always walk to the same spot, and give the command "Go Pee" or "Hurry up." If you set the timer on the oven for every 1.5 hours, then pick your pup up, and carry him outside, what does that teach him? But if you hook a leash to his collar, take him to the door, and lead him to the spot, then he has done it himself. You have to let THEM do it themselves.
Also remember it is best to avoid mistakes, so keep young pups 100% supervised. Avoiding mistakes works much better than disciplining mistakes. Quite often a young puppy may mistake your discipline, as "Uh oh, I better not let my owner see me pooping on the floor, I better do it when he isn't looking,” then you get a dog that poops in the house, but always behind your back, or the couch. So remember, rewarding correct behavior works tenfold over reprimanding incorrect behavior, as the puppy doesn't always know what he did wrong.
Even if the breeder did everything right to give the pup a good start on where to pee and where not to (by using a box with a potty station to teach the puppy that there is a designated area in which to eliminate), the new owners could undo all the pup has learned by constantly carrying the pup outside in their arms rather than taking the dog on a leash and letting HIM walk there on his own.
If the owners always pick up the pup in various areas of the house, how is the pup to learn how to signal when it has to do its business? A pup that is constantly picked up and carried outside to pee will not know how to tell the owners when it needs to go outside.
It will wait around (to be picked up) not knowing what to do and eventually will find a spot inside the home to pee, as he hasn't learned any more than to wait to be picked up every hour and be taken to the bathroom.
It is just so easy to pick up a three-pound toy puppy and carry him that we totally overlook what we are not teaching our puppy.
To teach the pup, it has to be made to walk to the door and out the door itself, teaching the pup HOW to signal the owners.
Eventually your dog will walk to that door and signal you in some way; some quietly wait looking at the door, some scratch, some have bells, some bark. There are many different successful methods, but they ALL begin with making your dog walk to the door.
For a new puppy, set your oven timer accordingly for age (see article on crate training for age and time of control) then hook up the leash and guide him outside.