mathematical proof of why the lava melts the bucket only from the outside
the item texture of lava bucket contains only 37 pixels of lava, while when its placed it contains 3840 pixels of lava. clearly, the lava when placed contains way more lava and thus more heat than the lava contained in the bucket. because lava is about 700 degrees centigrade, we can estimate that the bucket is about 1000C and the block is about 9000C. since iron melts at 1300C, only the block of lava melts the bucket.