How tight should soil be compacted

I'm not a gardener but I planted some seeds of food eaten from the kitchen 
(pears, apples, lemons, oranges, avocados, watermelon, pomegranate, etc).

I put the soil in typical plastic plant containers but I don't know if I'm 
supposed to pack the results tightly or loosely or mediumly. If I pack them, 
with a mallet I can easily triple the amount of soil and even if I compact 
them with my fist, I could easily double the amount of soil, so compaction 
certainly makes a huge difference in soil volume.

There must be an optimum amount of compaction though, for soil volume and 
for water sponging I would think that the experts already know about.

What's the rule on compaction when you dig the soil out of the ground with a 
shovel and then when you fill a dozen of these quart to half gallon sized 
black plastic planting containers?
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