Screen height (30 or 15 feet depending upon dry or wet conditions) and obstacle clearance are very different things. Light and heavy types are certificated to different schedules hence the difference.
Larger types are certificated with guaranteed performance in that, having reached the V1 they are going to complete the take-off to the end of the Net Take Off Flight Path (at least 1500 feet with all the obstacle clearance data).
Light types do not necessarily enjoy such good performance.
The 50 foot obstacle is simply a way of standardising performance data and producing conditions that are likely to be encountered.