Why Tribal destroyers had unequal funnels

Why did Tribal-class destroyers have one thick funnel and one much thinner funnel?

The forward funnel combined two sets of boiler uptakes, while the after funnel served only one.

The unequal funnel sizes followed directly from the machinery arrangement below deck. The forward funnel received exhaust from two sets of boiler-room uptakes. The after funnel served only one set and therefore did not need the same cross-section. The silhouette was a functional consequence of exhaust routing rather than a purely aesthetic design choice.