Flash tube size is generally related to flash power: more powerful strobes typically use larger tubes. That said, tube size alone isn’t a reliable way to judge whether a head is safe or well-designed.

From the answers, a smaller tube in a more powerful unit is not necessarily a major problem in practice. Lifespan differences between small and large tubes are unlikely to be dramatic, and flash tubes are usually considered replaceable consumable parts.

As for heat, more powerful lights generally generate more heat, and larger tubes can run hotter simply because more energy is involved. In normal studio heads, overheating risk is usually managed by the design of the light itself, such as built-in cooling fans and thermal cutoffs.

So in typical use, you normally don’t need to wait longer than the recycle time just to protect the tube, unless the manufacturer specifically recommends a duty-cycle limit. If overheating were a concern, modern heads are usually designed to shut down or protect themselves before damage occurs.