The warhead is typically 10-30% of total missile weight and most of the non-motor weight. A substantially heavier warhead on the same rocket motor greatly reduc

The warhead is typically 10-30% of total missile weight and most of the non-motor weight. A substantially heavier warhead on the same rocket motor greatly reduces missile acceleration, speed, and range. IR missiles must accelerate to ~Mach 2.5 over a very short distance to maintain lock and close the distance for the purpose of air intercept due to the short-range of the guidance. IR seekers are lightweight and compact, which lends itself to quick acceleration. This short-range performance profile can be maintained with a heavier warhead using a larger, heavier rocket motor. This has cost, weight, size, etc implications but that isn't a reason to not do it in isolation. The upgraded IR missile is still short-range but now it has a footprint similar to long-range radar missiles and those have a similarly large warhead. It erases the major technical advantages of IR missiles (cheap, light, small) without addressing their major deficiency (short range). You could build an IR missile with a heavy warhead but it doesn't make much sense. The quick acceleration requirement creates a lot of engineering pressure to reduce weight, which can only be meaningfully achieved by reducing warhead size.