The Evidence: Hill Giant Height Change = Correction from 2e to 3.Xe

TL:DR - An Ogre Isn't A True Giant

If you just looked at the two monsters standing side by side, the Hill Giant would be slightly taller and a lot stockier and bulkier in build.

1. Based on the progression of D&D editions, the change in height appears to be a correction to the bloat/choices in 2e. Hill Giants have always been a much tougher opponent than Ogres(this goes back to OD&D: an Ogre was Monster Table 4 Monster, a Giant was a Monster Table 6 Monster). This theme of comparative power (Lore?) remains consistent with the stats in 3.5e.

2. While Ogres may be of the Giant Class or Giant Type (usage varies with edition) Ogres are not True Giants. That status is reserved for: Hill, Stone, Cloud, Fire, Frost, and Storm Giants. The distinction goes back to OD&D/Greyhawk when Storm Giants were introduced, and the term "giant class" was introduced. Hill Giants were always the least powerful Giants(Monsters and Treasure).

   • An early mechanical distinction in why "Giant Class" matters had to do with Dwarves having an advantage fighting against them(ref Monsters and Treasure): Dwarves did +1 damage (Greyhawk)and took half damage versus giant and other large clumsy creatures like Ogres(Monsters and Treasure).

   • Also certain weapons were "Giant Slaying" -- the special damage bonus applied to giant class monsters. (Refs: Monsters and Treasure, Greyhawk).

   • Bugbears were added to "Giant Class" (Greyhawk). Gnolls and some Orc kin (1e AD&D). (Aside: Rangers got a damage bonus per level versus Giant class creatures. As with Dwarven advantages, there were some mechanical benefits based on that classification ...)

   I mention the above to point out that the "Giant Type" looks like the red herring that prompted your question. A better comparison is between your 3.5 Hill Giant and other True Giants to see if the power gradient lines up correctly.

3. Supporting points:(Stat format = HD/DMG/Height). The increased Giant Heights stand out in 2e. Compare that to the 3.5e Monster Manual: is the trend consistent?

   • As presented in 3.5e, the difference in weight appears to accurately reflect that the True Giant (a Hill Giant) is a larger, stronger, and much harder to kill creature than an Ogre that isn't a True Giant (though of Giant Class/Type). The Giant has 400 pounds of bigger/denser bones, more muscle, more body mass, more monster. (And he throws rocks at you!)
   • If you are trying to paint a picture for your players (per your comment): look at an NFL cornerback who is about 5'11 and about 180 lbs, and a fullback who is 6' and 240 pounds. They are both tough, and almost the same height, but the latter is far more powerful, stronger, and harder to knock down. When he hits you with a block, he hits you harder. Height isn't the end all and be all to power. With a 400 pound weight differential, the Ogre and Giant are NOT the same size when all dimensions and factors are considered.

Monster        OD&D             1e                2e                  3.5e 
(HD/DMG/Height) --               --               --                   --
Ogre       4+1/1d6+2/7-10'    4+1/1d10/9'        4+1/1d10/9'        4hd/Dmg/9-10'
Hill Giant  8/2d6/12'         8+1-2/2d8/10.5'    12+1/2-12+7/16'    12HD/Dmg/10.5'

Height(HD):   Hill        Stone     Frost      Fire         Cloud      Storm

OD&D       12'  (8)      15'(9)     18'(10+1)  12'(11+3)   20'(12+2)    24'(15)
1e         10.5'(8+1)    12'(9+1)   15'(10+1)  12'(11+2)   18'(12+2)    21'(15+2)
2e         16'  (12+1)   18'(14+1)  21'(14+1)  18'(15+2)   24'(16+2)    26'(19+2)    
3.5(SRD)   10.5'(12+48)  12'(14+56) 15'(14+70) 12'(15+75)  18'(17+102)  21'(19+114)

Bottom Line: this difference is consistent with D&D lore from its inception. The 3.5e MM(SRD) shows a progression and comparative power like what's always been in the game.

Nothing in the Shield Charge ability's description says that its movement is stopped by its attack during the Shield Charge. From its statblock:

Fireshield. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 22 (4d6+8) bludgeoning damage plus 7 (2d6) fire damage plus 7 (2d6) piercing damage.

[...]

Shield Charge. The giant moves up to 30 feet in a straight line and can move through the space of any creature smaller than Huge. The first time it enters a creature’s space during this move, it makes a fireshield attack against that creature. If the attack hits, the target must also succeed on a DC 21 Strength saving throw or be pushed ahead of the giant for the rest of this move. If a creature fails the save by 5 or more, it is also knocked prone and takes 18 (3d6+8) bludgeoning damage, or 29 (6d6+8) bludgeoning damage if it was already prone.

Judging from that, the shield charge lets the fire giant dreadnought move up to 30 feet in a straight line, including through the space of any creature that is smaller than Huge. The attack is essentially incidental to the move; if the target succeeds its Strength save, it is unaffected.

You could flavor this as if the character that makes the save is actually holding the fire giant dreadnought back just long enough to move himself/herself out of the way, or however else you want to describe it.

The dreadnought could, of course, choose to stop before the end of the 30 feet, since the Shield Charge doesn't specify that it must move the full 30 feet.