General :: Internal Memory Consumption - Hidden Files

I have only 150mb of internal memory on my phone. These are some items that consume critical amounts of internal memory space. When I download an app, it seems I am downloading an install package. The installed app itself takes up additional space. I believe the install package persists on the internal memory, invisible to the average retail user, and takes up internal space even after the app is moved to the card.

Ex: an anonymous app
android market listed size- 3.51mb (this must be the download size of the installer)
download and install on internal memory, size listed- 5.73mb
move to SD card, size listed- 2mb
move back to internal memory, size listed- 5.73mb

Further thoughts:

1. The install package seems to remain on the internal memory even though the app moves to the card.

2. Beyond the above example, I went through each and every item listed in my settings - applications - storage use - All.

I found 105 total items on internal memory and SD card all together. By adding up the listed "Total" size of each (yes, I went through each and every one), I arrived at approximately 109mb. The phone lists internal memory use at 135mb. I would expect the internal memory use to be LESS than 109mb because some of the apps are on the card. My assumption is that much of this extra internal storage is taken up by all of the installers sitting invisible on the internal memory.

3. Based on observing the change in the Internal Memory usage when I delete app "Data", I believe the app data is being written to the internal memory even if the app is on the SD card.

Why isn't the install package deleted after the app is successfully installed? Is there an app for that?If I uninstall the app while it sits on internal memory, does it also delete the install package?If I uninstall the app without first moving it back to the internal memory, does it delete the uninstall package? How can I reclaim this internal memory space for actual use??