Feature importance ratio
In fact, this means that the quality of the feature f1 is very high. Usually, you should be worried if you get all the features giving the same importance level, and that level is low.

It is very important to understand that random forest does two level of randomization: at the data level and the feature level and it is very difficult to over fit.

Since you have this large number of trees in the forest, I do not think you have over fitting problem.