YJ-18 needs the 9m length because it is hot launched with a booster which in turn needs the concentric canister launch exhaust as well which adds additional length.
On the 052D, the two rear 8 cell modules in the bow 32 cell VLS section is thought to be 9m long alone. On 052D we've only seen YJ-18 launched from those 16 VLS cells.
Regardless, you haven't provided any evidence to suggest that the UVLS is limited to supporting payloads of 1.5 tons, so a retraction from you would be appreciated.
Cold launch for HQ-9B needs extra bottom ejactor which could occupy more space than YJ-18 booster.