import collections


def volume_versus_size(clusters):
    """Calculate volume of clusters respect to its size.

    Args:
        clusters (list): list of clusters

    Returns:
        dict: {size (int): volume (list of floats)}

    """
    stats = collections.defaultdict(list)
    for cluster in clusters:
        # initial container is empty
        stats[cluster['size']].append(cluster['volume'])
    return stats