See https://github.com/plotly/plotly.py/blob/master/plotly/tools.py#L1336. Large font leads to overcrowding when plotting multiple subplots with titles.
It would be better to have make_subplots accept a kwargs parameter like font_size=8 or, more general, to have a kwargs parameter like font=graph_objs.Font(size=8, color=...)