Why do some apps feel “heavy” while others feel “light”?

I think the "heavyness" factor commented is a combination of the following:
  • Performance. Speed of computing. This is a technical factor (you can measure the number of milliseconds it takes your web page to load or an algorithm to produce results). Companies such asGoogle and Amazon have checked that minimal delays in page load time affect user satisfaction.
  • Responsiveness. The degree in which the system does not get "frozen" while computing. A good example is how Google maps loads tiles on mobile devices: the user can move around while the new tiles are being loaded. Depending on the connection speed, tiles can take some time to load but UI responsiveness avoids users to get frustrated with the wait.
  • Seamlessness. The lack of barriers to the user flow of actions. These barriers include alerts, confirmation boxes, instructions on how to format data to the user, etc. For example, Google Chrome makes a good use of non-blocking dialog bars.
The above terms appear in the "best practices" section of the Android developer guide. I use the term "non-blocking interactions" to refer to "responsiveness" (non-blocking the user while computing a result) and "seamlessness" (non-blocking the user flow of tasks) since I consider that the "perceived speed" is more related to these factors that the real speed.