This is technically true, but in practice planes are designed for or or two seating configurations. For example you're always going to have wider seats in an A320 than a 737 because no airline is going to configure their 737s 5-across.
To the best of my knowledge, no airline has configured 787 economy anything other that 9 across. There's not actually a direct comparison to Airbus, the A330 is slightly narrower but is usually configured 8 across, whereas the A350 is slightly wider and is also usually configured 9 across.
So as a general rule, and subject to exceptions such as the proposed 11-across A380, Airbuses will have wider seats in economy than Boeings.
To the best of my knowledge, no airline has configured 787 economy anything other that 9 across. There's not actually a direct comparison to Airbus, the A330 is slightly narrower but is usually configured 8 across, whereas the A350 is slightly wider and is also usually configured 9 across.
So as a general rule, and subject to exceptions such as the proposed 11-across A380, Airbuses will have wider seats in economy than Boeings.