[Bareon]

Hmm, well... You're going to need a few lessons from the web about how to downsize a mesh and make a final design. Too many Verticies can really clog a game up.

Retopology
Normal Mapping
Subdivision Surfaces
Soft Body Dynamics
Rigging
Animation Transitions
Weight Painting (skeleton Rig)
And mayhaps a little Unity C#

When you make a model for BE, you have to design it at its Maximum size, then rig it up.
Possibly by using a few Shape Keys starting from the smallest and working back up to the largest size.

One has to remember that when using sliders to affect shape, you must do them in the order that you design them to work. The First shape will affect every shape afterwards, unless you can fully disable the previous one with a good transition before bringing the second slider up.

You may be able to work with a single shape-key if the form does not change, but most masses change angle or weight distribution as they get larger.