In this example [itemsizing](https://plot.ly/python/reference/#layout-legend-itemsizing) attribute determines the legend items symbols remain constant, regardless of how tiny/huge the bubbles would be in the graph.

import plotly.graph_objects as go

fig = go.Figure()

fig.add_trace(go.Scatter(

x=[1, 2, 3, 4, 5],

y=[1, 2, 3, 4, 5],

mode='markers',

marker={'size':[10,9,3,6,4], 'sizemode': 'diameter'}

))

fig.add_trace(go.Scatter(

x=[1, 2, 3, 4, 5],

y=[5, 4, 3, 2, 1],

mode='markers',

marker={'size':[700,2000,400,1000,750], 'sizemode':'area'}

))

fig.update_layout(legend= {'itemsizing': 'constant'})

fig.show()