where did that shape come from and when

The first baseman's glove is the biggest glove on the field.
Its also know as a mitt.
Its bigger so that they can snag the ball thrown to first easier. To dig balls out of the dirt, stretch for balls and etc.

First baseman's glove 100 embodies the same invention as fielder's glove 10, but is different in size, shape, and padding configuration to specifically meet the needs of the first base position. First baseman's glove 100 has an outer shell 102 composed of a front surface 104 and a back surface 106 joined together at various parts by lacing 108. Front surface 104 and back surface 106 are not joined together at the lower portion of first baseman's glove 100, so as to form a glove opening 110. Outer shell 102 is formed to include a palm portion 120, finger portion 130, and thumb portion 140. A web region 150 is located between finger portion 130 and thumb portion 140. A web-reinforcement patch 152 is attached over the intersection of the lower portion of web region 150 and outer shell 102. It should be recognized that web-reinforcement patch 152 is not limited in size and can be situated on first baseman's glove 100 to cover a variable region immediately surrounding the intersection of the lower portion of web region 150 and outer shell 102, including the areas directly above, below, and on both sides of the intersection.

First basemen have the job of getting a runner out when a ground ball is induced in the infield. A 1B glove is designed specifically to scoop up balls that either are poorly thrown by a pitcher, catcher, 2B,SS, or 3B or catch balls that are off target by a bit. Albert Spalding in 1877 had recently transfered from pitcher to 1B where he had implemented longer fingerless gloves and more padded ones to reduce shock. As years progressed it kept steady up until big drawn out webbings were put in use in the 50s. This made it so the 1B glove could be expanded without leaving such a big void between thumb and index finger. Though they actually "had" webbings in the 1900-1940's they werent all that visible (they were pretty much just lasings).

It helps with getting badly thrown balls from other infield players.

easier to catch and plus first basemen rarely have to pull the ball out of their glove quick and throw so you want a big deep pocket

all answers had the right explanation but partly the rounded part on the backside of the glove was designed not only for digging balls out of the dirt but for also applying a tag when the pitcher throws over since first base is where 98% of the pick off throws go to...the rounded part gives the first baseman just that much more advantage on quick tags.

The first baseman's glove is longer and bigger for a two reasons. First, it is easier to scoop a ball out of the dirt with this glove than a normal infielder's glove. Second, when the first baseman streches, it adds a few inches to his arm. This allows him to reach farther and he will have a better chance at getting a runner out.