How to fix the sinking area surface after decimate? I have a 3D model which I am going to use it in a website but the size of the model is way above acceptable range for online applications. So I tried to decimate the model, but its surface sinks in. how can I fix it!?

Original Rendered Model



After Decimating
Another approach would be to bake your high-res model to a displacement map which you apply to a low-res mesh afterwards.
I found a video tutorial how to do that:

https://m.youtube.com/watch?v=l1ASzhU9O-A