G in G scale comes from the German word Gro, which literally translates into Big. As you all know G scale is one of the most popular scales due to its collectability due to their size. It translates to 1:22.5 of the replicated train and run on a gauge of 1.75 inches. As you noticed they are huge compared to other popular sizes.
O scale trains are 1:48 of the replicated train and run on gauge of 1.25 inches. Due to their size you can see lots of details, especially in well crafted pieces. Their detailing makes them sit on top of the list. They owe their ranking due to the amazing detailing they have.
HO is one of the most popular scale in model making of trains. It translates to 1:87 of the replicated train and it runs in a gauge of .625 inches. Even though they are small, they always keep a lot of the details magnificent trains have to offer. HO is the last line of scaling before some details get lost in transition. Smaller the model gets, less the details it has. HO scales are so popular due to their size since it is relatively easy to start for people who are new to the hobby.
As we approach bottom of the list, the smaller scales get. N scale trains are popular because they are so small you can build them and insert them into anything you can make at home. Their scale is 1:160 of the train and they run on a rail gauge of .375 inches. With all of these pros comes the cons. Because they are so small detailing often gets lost in manufacturing. So if you are into trains detailing in collecting, N scale might not be the right choice for you.
Here comes at the bottom of the scale, the Z Scale! Being one of the smallest in our popular scales list they come in 1:220 train scales. Z scales use a gauge of .25 inches, as you can tell they are really small. They are often used in making dioramas thanks to their size.