The clearest explination I can give is that American Mecha (see BattleTech and MechWarrior) tend to favor hard science fiction, so their mechs are literally walking tanks, their strategic value based around the fact that they can carry a large assortment of heavy weapons and have the manuverability to fire them in ways that tanks can't.
Japanese Mecha are much less restrictive, and designed around almost science fantasy, in ways that could never feasibly work according to our current level of science. Mecha from series like Macross and Gundam depict their mechs as being able of moving with more speed and grace than the humans that pilot them, wielding everything from gigantic beam cannons to gigantic combat knives. They're essentially just enormous forms of power armor, enhancing their pilot's movement instead of being a vehicle that walks, in ways that physics simply wouldn't allow.
Consider how fast an elephant moves. It can get up a good forward pace, but if he tried to make a sharp turn he'd likely trip, and seriously damage himself from all that weight falling from that great a height. Now consider a 50 foot tall mech, probably weighing several a few dozen tons, trying to perform martial arts.
Even when the Japanese try for hard science mecha, they fail at it. Armored Core and Front Mission Evolved are two good examples of this - their mecha lean towards American styles (with their mechs being more about the kind of firepower they can carry than being giant power armour) but then they have access to energy blades and both mecha types can slide across the ground at very high speed without any attempt at realistically explaining how they can do so (they don't have wheels, any sort of hover technology, just a big booster rocket sticking out of their back that somehow maintains their perfect balance, maneuverability and speed). They skate about like they're on ice, never tripping up or being affected by the recoil of their own weapons.
That isn't to say that American mecha don't branch into suspension of disbelief. MechWarrior utilizes jump jets which in earlier versions were literal flawless boosters that allowed for insane manuverability for the mechs. More recent editions have toned it down a bit so that the rockets act like literal rockets; slow, fuel-consumptive, and not intended for in-combat situations (they're more for accessing a tactically advantageous high-ground).
If you need more info than that, just search the internet. Focus on Macross and MechWarrior. Both have similar origins (MechWarrior was kind of shameless about ripping off some of Macross' designs, what can I say, it was the 80's) but the directions that both franchices took reflect their cultural differences.
That's why MechWarrior 4: Mercenaries was a kind of "mech simulator" where you had to manage things like heat sinks and armour tonnage while Macross 7 was about transforming jet-robots fighting faerie-monsters with the power of music.
Yeah. That happened.
Sooo... really, take your pick.
You can check out Neon Genesis Evangelion, too, but that's about children in pools of embryonic fluid fighting in cyborgs cloned from an alien version of Lilith from the Biblican texts. Also the "mecha" are attached to umbilical cords. And one dude masturbates over his comatose friend or something.
I'm only biased because seriously, WTF Japan?