Propositions do not start long. They get long.
The first version, written by one or two people early on, is usually the best one anybody will produce. It is short because it was written before anyone else had to agree to it. Then it goes round the business, and what comes back is not disagreement. It is addition.
Nobody says no to a proposition. That is the mechanism. What happens instead is that each department reads it, finds their contribution absent, and asks for a small inclusion. Product notes that it does not mention the platform. Sales notes that it does not mention service, which is what they actually win on. The regional MD notes that it reads as very UK. Legal notes that "fastest" is a claim. Each request is reasonable in isolation, each is easy to accommodate, and none of them is a rejection, so there is no moment at which anyone can push back on the direction.
By version nine it is forty words, it contains everything, and it means nothing. And because it accumulated by consensus, everyone signed it off and nobody likes it.
I have been on the agency side of that process many times. It is not a failure of client intelligence. It is a failure of process design: you have created a system where the only available move is addition, so addition is what you get.
Two things fix it, and they both have to be agreed before the first draft circulates.
Name the decider. One person, senior, who owns the proposition and can say no without a meeting. Not a committee chair. An owner. Consensus produces length because consensus has no mechanism for refusal.
Set a word limit and treat it as fixed. If the proposition is capped at, say, twelve words, then every addition requires a subtraction. This converts the exercise from accumulation into trade, and trade surfaces priority almost immediately. The conversation stops being "can we mention service" and becomes "is service more important than speed", which is the conversation you needed in the first place.
There is a third thing, harder to schedule. Somebody has to be willing to tell a senior stakeholder that their department's omission is deliberate. Not an oversight to be corrected: a choice, made on purpose, in service of the business. That is an awkward sentence to say to a colleague, which is why it is usually easier to bring in someone from outside to say it.
The wider point is that length is a symptom, not a fault. A long proposition is telling you that a decision has not been made yet and is being deferred through inclusion. Cutting the words without resolving the underlying priority just produces a shorter document with the same problem.