You can use an app that is aware of the zoom so can get rid of labels that are too big.

Normally tho I have seen that big labels have been done real faded so when zoomed in then they are a lot less conspicuous.

Another option is to keep all labeling on different layers or even better, to dynamically label the map depending on the zoom and recreate several versions automatically from that.

Its really something you need to plan for tho so retro fitting a solution is rarely easy or elegant.