Canada is smaller than Russia.
This is perhaps one of the more difficult grammar aspects to generally get an idea in your head because it all depends on the contect of the situation that the statement is being used.
Canada is a big country. We all agree with that. But Russia? Russia is a big country. It is bigger than Canada. Canada is also a big country, but it is not as big as Russia.
From this example, the context is clear. We do not consider Russia and Canada to be small countries, so we would not usually say Canada is smaller than Russia - rather, Canada is not as big as Russia. But if, for example, we are talking about a group of big countries rather than just two examples, then we might say As for the USA, Russia, China and Canada, China would be the smallest of that group- but it's still a big country.
But if the countries were, for example, Belgium and Luxembourg, we would consider them as small in area, and so would be considered as small countries. So we are more likely to say Luxembourg is smaller than Belgium. But in the context of a number of small countries, Belgium would be the biggest when compared to Luxembourg, Leichtenstein, San Marino, Malta and Cyprus. But it is still one of the smallest when we talk about it in the context of Europe.
Here is another (with apologies to those people who live in Brisbane, Australia, and Pyongyang in North Korea): would it be more correct to say Brisbane is not as ugly as Pyongyang or is prettier than Pyongyang? According to a particular website, neither of these cities are what you would call pretty, so you would be more likely to use the first example.
In the same way, Venice and Prague are cities that you certainly would not call ugly. So when comparing the two, what are you most likely to say? Prague is prettier than Venice (but Venice is still a pretty city) or Prague is not as pretty as Venice (but Prague is still a pretty city).
Some more examples which I hope will explain how these comparatives work:
The Bugatti Veyron is faster than the Ferrari. (But the Ferrari is still a fast car - but it is not as fast as the Bugatti Veyron)
The Ferrari is more expensive than the Lambourgini. (But the Lambourgini is still an expensive car)
The River Thames is longer than the River Trent. (In the context of the UK, both rivers are long, but the Thames is longer)
The Market Square is busier than the shopping centre. (Both places are busy, but the shopping centre is not as busy as the Market Square)
The later Star Wars films were not as good as the early ones. (Both were good, but the early films were better)
But compare when we put some of these examples in a different context:
The later Star Wars films were bad when compared to the early ones. (The early Star Wars films were good or better; the later ones were not!)
Or if you really do not like these films:
The later Star Wars films were worse than the early ones (In your opinion, the early Star Wars films were not very good)
Or maybe you do not like scenery, views or architecture:
Prague is uglier than Venice. (In your opinion, both cities are ugly)
Or if you come from a country where many of the rivers are hundreds of kilometres long:
The River Trent is shorter than the River Thames. (from your context - you see rivers as long - both rivers in the UK would be considered short).
So when some examples are clear when making comparisons, others may not be so when the contexts are changed. In many examples, that really depends on how you view the objects of your attention.