This is Gustav III of Sweden and his brothers in 1771. Sleek, fitted, and very suave. But at the beginning of the century, fashion was still retaining some of the bulkiness and heavy feeling of the baroque period.
These two gentlemen are from an illustration from 1725 (I apologize for the size, it's the best I can do). Their coats have a huge amount of fullness in the skirt and the cuffs are enormous, lending a sense of heaviness to the garment. If you've ever studied art history, you know that one characteristic of 17th century art is that it's very heavy. Eighteenth century art, in contrast, has a very light, frothy feel. This translates to fashion (as well as architecture, music, all art forms reflect each other, it's really fascinating). These styles retain the heaviness and bulk of the 17th century, whereas the clothing worn by Gustav and his brothers has the light feel associated with the 18th century. Another thing to notice is that the coats also have the ability to close all the way.
This gentleman from 1724 has his coat open, so that you can see his waistcoat. It continues in the fullness of the coat, and goes down to his knees.
Our next dashing gentleman is from 1746. His waistcoat is also long, but the fullness in the skirt of the coat has gone down, and his sleeve cuffs have gotten smaller. In all, his coat is starting to get a sleeker appearance.
Here we've made it to 1753-4. As you can see, things have definitely slimmed down. The sleeve cuffs have shortened considerably, and in doing so have lost a lot of their fullness. Although he's sitting down, you can see the the coat skirt is significantly narrower. The coat itself has started to slope back at the bottom, and so it can't necessarily button all the way down. This is a great example of a really common phenomenon in fashion- something starts out as functional and becomes non-functional but very fashionable. Buttons and buttonholes started as function, at this point they are mere decoration. His waistcoat has also gotten shorter.
Here's a gentleman from 1771 who is standing, to give you a better idea of what I mean.
Note how these changes give a much lighter, more elegant appearance.
Our final contestant is from 1795. His sleeve cuffs are gone, the waistcoat ends at the actual waist, and his coat slopes back considerably, creating what you might think of as a tailcoat. Men are now ready to enter the Regency period.
Please note that these are just general trends, and individual styles my vary.