That's only the real robot genre of mecha. Those are based more in realistic settings. The mechs tend to be cold lifeless machines, and in a few cases might have built in AIs. The setting will have consistent internal logic even if it includes such things as space battles and beam weapons.
Mobile Suit Gundam was the iconic series that established the genre. Most of the fancy tech was due to the discovery of a new substance that interacted with nuclear radiation. The mobile suits were powered by compact reactors made possible by this discovery. The high amount of unusually close combat and lack of guided missiles (in other words the reason for all those cool action scenes that included melee combat) was explained by the dispersal of the new substance jamming radar over a large area.
Super robots on the other hand throw realism out the window. They often have an actual limited personality that chooses their pilot. They also tend to power up based on things like fighting spirt or the emotions of their pilot.
Take the recent Voltron for example. The giant robot lions only allow people with the right mindset to pilot them, and act on their own from time to time. They also generate weapons from out of nowhere that make little logic sense. I mean how does the green lion have a cannon that shoots seeds that instantly grow and consume their target in vines?
Then there's the whole combining into Voltron, which requires the pilots to be in sync to achieve. Voltron then is controlled by five people at once, and is pretty much unstoppable if the pilots have enough willpower.
So super robots are pretty much magical mecha. Thus they can pull out the same bullshit that magical girls can.
Just Comparing both Genres, I can deduce that to balance out their respected powers, we need to make a limit to how powerful they get.
Magical girls are created with cute and overly powerful magic that can eradicate any foes with ease with their powers.
Mechs are more I can say, giants or suits of steel to fight off enemies with powerful scientific energy weaponry to fantasy mana pulsation attacks.
I can make an analysis given both genres:
Magical girls
*Light Armored
*Mostly Strength Based to Magic Based
*High Firepower
*Power Scale: Low to High to Godly
*Lacks in Speed(Given that they tend to show off their powers
*Low Stamina(Given that most MGs are from ordinary girls)
*Lack of Combat Skills
*Lack of Experience
Mechs
*Light to Heavy Armored
*Depends on the Mech(Strength based, Speed Based, Glass Cannon.... Etc.)
*From Suits to Giant gundam of steel
*Power Scale: Low to High to Near Godlike
*Many sets of weaponry to High Firepower
*Needs Energy(Battery to Mana)
*Experience Pilots
*Highly Competent Soldiers
*Not built on Small area fighting and mostly on fields
*Can be Damaged but repaired(Also regenerate if made of special materials)
*Has AI to help the pilot for any battle strategy or plans.
*Has a Core that operates the Mech.
This is the info I collect given the two comparisons. Magical Girls are quite difficult to gauge but given the history of authors that made them, we can calculate their power scale to almost godhood. In comparison to Mechs, I never felt like some mechs are godly but near one persay so the advantage goes to the MGs.
Durability? Hands down the Mecha wins by default. Magical girls aren't suited to take on long fights while Mecha while having such weaknesses, can still overpower a magical girl given the experience, skills and their energy reserves of the mechs.
Strengths? MGs wins that one with godly scales than Mechs.
Speed? Mechs win because magical girls tend to overpower their opponents and given their stamina, they aren't that much mobile given their small and short structures.
Battlefield? MGs wins due to the Mech's giant frames and any battle in small fields are harder for them to fight and aren't able to show off their skills. Plausible for larger fields but everybattlefield must always take advantage on any terrains.
Combat prowess? Goes to the Mecha. From swords, guns, snipers, energy beams to even silence weaponry. Magical girls can only fight with either physical or magical attacks and aren't that well versatile. Plus, most MGs are young girls who aren't that much experience in fighting while Mechas has mostly experience soldiers who may have many pass history at its back.
I think this is my own analysis but you can give your inputs about my research.