As part of the Second Reinforcement Program of 1934 the Navy wanted a carrier design that future designs would be based on.
Soryu was designed based on experience with the Ryujo. The Soryu was the first Japanese carrier to be designed from the keep up. The design for the Soryu was still under the London Naval Treaty restrictions however when the Hiryu was to be built it wasn't.
The Hiryu's forecastle deck was one level higher than Soryu's. After the 4th Fleet suffered storm damage it was decided to build Hiryu with a stronger hull and beam to improve stability.
There were three elevators serving the two hangars on the Soryu and the Hiryu.
The upper hangar was 520' long and the lower 420'.
Hiryu's bridge was on the port side near the middle of the deck and Soryu's was on the starboard side more forward. Hiryu's placement caused problems with air currents and reduced the area for aircraft landing.
The funnels on the Soryu were just in back of the bridge and went horizontally and then downward.