Why an indirectly bound navy might observe treaty limits

Why would a ship be kept within the limits of a naval treaty that did not directly bind its navy?

Because exceeding the limit could trigger escalator clauses for other powers and produce a broader, disadvantageous round of naval expansion.

A navy could have reasons to respect a treaty limit even when the agreement did not bind it in the simplest direct sense. Going over the threshold might activate escalator clauses available to other powers. Those powers could then build larger or more heavily armed ships in response. One design decision could therefore change what every other navy was permitted to do.