A few things to consider -
One > is not to have too much bass in the first place - a balanced (in terms of EQ) sound will sustain better across different listening environments and speakers (even tho massive bass will sound awesome on good speakers!).
Secondly > there are some frequencies that 'hint' at bass - brining these out will create the psycho-acoustic understanding that there is bass sounds there that arn't actually played by the speaker. I'm not good enough to tell you exactly HOW to do this step by step - I can only achieve this by tinkering and listening and testing at the moment (sorry for no hard tips here).
Thirdly > this is probably the most practical and useful tip. When it comes to small speakers I think that it is really, really difficult to balance the audio so it is useful (dialogue is clear and relevant information is provided to the player) while at the same time providing an awesome aural 'experience'. This is just not entirely possible on tiny 'tinny' speakers. So what we can do is go for a psychological effect of bigness - rather than making a big sound - we imply bigness with reverb and the idea of bigness. Examples of this could be twigs snapping or tree branches snapping, a rumble that goes on a little after the actual impact of the giant footstep has finished and so on. A good example of this is the melee weapons in Left 4 Dead 2 - the guitar and the cricket bat and golf club all have sounds that are totally not realistic in terms of whacking zombies (whatever realistic means in that situation!) but they all 'sound' like the object in question and provide a very clear psychological effect.
Hope the project goes well!!
All the best,
Paul 'Volumetric' Nunes