Ex-S is back heavy because those are capital ship-grade boosters attached to it's back. The design is meant to get in, disrupt enemy lines, and get out. That's why it has fuel pods attached to its shoulders in Ex-S mode.
Also, Gundam Sentinel takes place entirely in space. Therefore, weight has no bearing on the design of the space use-only Ex-S Gundam. Calling it back heavy has no meaning, unless you have something against logical design.
Well, let's think about that. Master Gundam, Ex-S Gundam, S-Booster Gundam, Wing Gundam Zero Custom, Wing Gundam ver. Ka... versus Nobel Gundam, Temjin, GP03S, GM Type C, Zaku II F2, Tallgeese, Deathscythe ver. Ka, Sandrock ver. Ka, Devil Gundam, Taurus, Virgo, Mercurius, V Gundam, V2 Gundam... the list goes on.
Most of Katoki's designs aren't back heavy. It's the back heavy ones that people happen to find most notable--which says something in and of itself.