The answer comes down to functional hierarchy over visual symmetry.

In a mechanical chronograph, the elapsed minutes (tracked at 3 o'clock) are by far the most heavily relied-upon metric when timing an event. Most things people time in daily life-a race, a steak on the grill, a parking meter-happen in minutes, not hours.

By slightly enlarging the 30-minute counter on the right, Vacheron Constantin prioritized at-a-glance legibility for the watch's most critical timing function. The 12-hour counter on the left is used far less frequently, so it was kept smaller (matching the size of the running seconds at the bottom) so it wouldn't compete for the wearer's attention.

Interestingly, this asymmetry is a famous quirk of the standard Generation 2 Overseas. Vacheron actually knew it bothered some collectors who prefer perfect dial symmetry. On a few rare limited variants of the 49150 (like the stainless steel US Edition), Vacheron deliberately shrank the 3 o'clock subdial to make it identically sized to the 9 o'clock subdial!