The Key Value API Explainer describes that there is a 2MB size constraint on responses from the Trusted Key Value server. This 2MB size limit is applied across all keys such that the total size of the response for all keys returned from the trusted server cannot exceed 2MB.

This seems like a reasonable size limit for a single interest group, however it becomes unclear how this constraint could be applied when Trusted Bidding Signals requests are coalesced across multiple interest groups with the same trustedBiddingSignalsUrl.

As the Fledge explainer describes "The [Trusted Bidding Signals] requests may be coalesced (for efficiency) across any number of interest groups that share a trustedBiddingSignalsUrl (which means they also share an owner)."

If the Trusted Bidding Signals requests were limited to a single interest group, then it would be easy for the Trusted Server owner to understand the size constraint for a single key's value: 2MB / number of keys per interest group.

Once an unknown number of IG's keys are coalesced into a single request we can no longer accurately assess a size constraint on a per-key basis to ensure that all of our key's values can fit within the 2MB limit. If we exceed the 2MB limit for a response keys are dropped at random which means we, the DSP, lose control over how we are participating in auctions.

Some possible solutions to this problem could be:

1. Prevent the browser from coalescing requests and make a single request per IG.
2. Apply the 2MB limit on a per Interest Group level rather than the total response size.

I'd be interested in hearing what could be done to address this problem in Fledge.