def slim_frame_data(frames, frame_allowance=25):
    """
    Removes various excess metadata from middle frames which go beyond
    ``frame_allowance``.

    Returns ``frames``.
    """
    frames_len = 0
    app_frames = []
    system_frames = []
    for frame in frames:
        frames_len += 1
        if frame.get('in_app'):
            app_frames.append(frame)
        else:
            system_frames.append(frame)
    if frames_len <= frame_allowance:
        return frames
    remaining = frames_len - frame_allowance
    app_count = len(app_frames)
    system_allowance = max(frame_allowance - app_count, 0)
    if system_allowance:
        half_max = int(system_allowance / 2)
        for frame in system_frames[half_max:-half_max]:
            frame.pop('vars', None)
            frame.pop('pre_context', None)
            frame.pop('post_context', None)
            remaining -= 1
    else:
        for frame in system_frames:
            frame.pop('vars', None)
            frame.pop('pre_context', None)
            frame.pop('post_context', None)
            remaining -= 1
    if remaining:
        app_allowance = app_count - remaining
        half_max = int(app_allowance / 2)
        for frame in app_frames[half_max:-half_max]:
            frame.pop('vars', None)
            frame.pop('pre_context', None)
            frame.pop('post_context', None)
    return frames
