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It is well known that the minimal space overhead for a non-immediate object in Ruby is 20 bytes*1. But the overhead will be much higher if your object happens to have instance variables (read: almost always).

Take for instance

class Stupid; attr_accessor :foo end
class Stupid2; attr_accessor :foo, :bar end

o1 = Stupid.new
o1.foo = 1
o2 = Stupid2.new
o2.foo = 1
o2.bar = 2

o1 will take around and o2 bytes, i.e. for objects with under 55 instance variables, the memory footprint is around 100 bytes plus 20 bytes per instance variable --- that's quite heavy.

The exact numbers depend on the behavior of malloc regarding memory alignment and overhead. Doug Lea's malloc, in wide use, takes 8 or 4 bytes per chunk.

Here are some other overheads for Arrays, Hashes, etc. (+M denotes the malloc overhead per allocated chunk, normally 4 or 8 bytes):