Let's think about why that is.... Unlike an ocean ship, a river vessel is very limited in size. It can't be too long or too wide as it has to fit into the locks, it can't be too high as it needs to go under bridges (and in fact most vessels have a hydraulic wheelhouse which is lowered to enable it to pass under low bridges), and it can't have too much of a draught as the rivers can be quite shallow.
So by the very nature of things, space is very limited, and depending on the river, capacities range from tiny numbers (sometimes as low as 20 - 30 on the more exotic rivers) to around 130 on the larger rivers, 190 on the Viking longships, or higher numbers on the huge Yangtze vessels.
The cabins are usually spread over three decks, and unlike on an ocean ship, you don't need to make allowances for the movement of the vessel by selecting something midships - movement is minimal. On the other hand, noise and vibration can be high, so you will probably want to steer clear of the back of the ship where the engine is usually located.
On the lowest deck you will find the cabins with small windows which look straight out over the water - what we refer to as the 'duck's eye' view. These cabins can be quite dark, especially when you are moored up next to a bank or pontoon, and the windows cannot be opened. Tauck have a unique 'loft' design for these cabins which allows more daylight to flood in over a raised mezzanine platform. Higher up, you will find cabins with larger windows, sash windows, French windows, French balconies or walk-out balconies.
Remember that even if you have splashed out for a balcony, you will often find you are moored up adjacent to another vessel, giving you a view straight into someone else's bedroom. On the other hand, many people enjoy being able to open the window and enjoy the fresh air. This accommodation can be more spacious, but in river parlance, a relatively modest cabin size (by ocean standards) of 250 square feet, for example, is often referred to as a suite, and on many vessels, there is nothing larger on offer. A bathtub is very rare.
If you compare like with like pricing by cabin size, you will often find the higher rated operators are excellent value for the more spacious accommodation. All in all, though, don't expect to spend very much time in your room - you will be ashore, out on deck or in the panoramic lounges enjoying the excellent views.
Many operators offer a limited number of lead-in cabins with no single supplement, so look out for these offers, and plan ahead to snap them up if you want a specific departure date or itinerary.
Finally, you should also consider the cabin layout. More modern vessels tend to have beds that can be configured as twins or doubles, but this is not always the case, so ensure you have discussed this at the time of booking. And on some vessels the beds face the window, giving you great views, so this is also something to consider.