Why do people say that pit bikes are cheap bikes and not real bikes. is that true. are dirt bikes better?

also, is a 125 pit bike bigger or smaller than a regular 125 dirt bike

pit bikes are really just that they are designed for use in the racing pits and nothing more...
they are usually a cheap chinese made copy of honda engine thrown into a simple cheaply constructed frame with lots of weak alloy components
they are not real off road dirt bikes and are really only toys
though some of them are now under more stringent control and are made in a maner suitable for use on the road but still not as dirt bikes.
They were and are purely to act as a means of fun and transport for pit crews.
a 125 cc engine is a 125 cc engine they are not bigger or smaller. but a pit bike is a small compact machine where as a dirt bike is really a stripped down motocrosser so they are physically bigger, though some of the manufactures to increase the market have advertised pit bikes as dirt bikes but this is really incorrect they are really toys and nothing more.
do your self a favour save your money and buy a real motocrosser or a trials bike from a reputable manufacturer
If it is a cheap dirt/ field bike you want as a bit of fun in a field or garden or paddock buy an old road bike which is no longer fit for the road. Strip off every thing you don't need and sell the bits on Ebay, use the money raised to improve the bike and either run it till it goes no more or until you get bored. then buy another and sell the remains for spares
your doctor and surgeon will probably thank you for saving them the task of extracting the bits of broken casing from your temple.\
The reason being is I've seen these pit bikes literally collaps under the weight of a small teenager as they try to emmulate motocross riders on them, not stunts just running them over bumps in a field. One kid wearing full protective clothing enden up having to have the spokes of a broken front wheel removed surgically from his arm and shoulder as he pulled a little wheelie to impress is mates and on landing the front wheel litterally exploded, the metal of the rim being thinner than that used on sidings or cladding. incredible really that it held together while he was jus riding it.

Not really a good pit bike is fine "if you want one" the junk is those from China and they are pure junk

It would be fair to say Chinese pit bikes are junk, but you could say the same for Chinese dirt bikes as well. There is no "real" category for pit bikes. It is just a coined term referring to the bikes that us racers ride around in the pits. The Honda CRF50 and Kawasaki KLX110 are two of the more popular Japanese bikes that folks use as pit bikes. By definition these are kids off road bikes, but when us big kids ride them in the pits we call them pit bikes. These two bikes, like all Japanese bikes, are of course quality motorcycles. A 125 pit bike is smaller than a 125 dirt bike by quite a bit. In recent years many Chinese companies have started to sell pit bikes, pocket bikes and dirt bikes in the US. These are cheaply made bikes and are the ones folks are referring to when they say pit bikes are cheap. A company called BBR actually sells modified "kids" bikes like the CRF50 that have been modified for adult pit bike racing and these bikes cost in the neighborhood of $10,000 which is anything but cheap.

Because they are toys and should not be on the street ever and children who do not have any sense of the rules of the rode should never ever have one.

pit bikes are fun, it's kinda like buying a portable TV, not the best picture or sound, but fun anyway...

hardleys are like 18" console B&W tv's that weigh 800 lbs, yet their owners claim that they are better than a 42" plasma in 1080P

Q #1 They are cheaper than a real bike
Q #2 Yes it's true they are for the most part cheaper
Q #3 Dirt bikes are better for riding in dirt
Q #4 A 125 Dirt bike is most likley bigger

Pit bikes are not intended for racing more just for fun that could be the reason that some people say they are 'not real bikes'. The size of the bike depends on the brand most of them are very close in size, The size of the motor of course will be the same between a pit bike and a race style dirt bike.