First of all, THE MP7 and the MP5 are basically SMG's, search it up bud. They were both invented by Hechler & Koch, German submachine gun.
MP stands for Machine Pistol.
MP5 is a 9mm submachine gun that is widely used.
MP7 has a 4.6x30mm cartidge.
Thus the reason why they are such lightweight, not to mention, it's called combo'ing.
The MP5 is also made using metal stamping techniques, also seen on the AKM. That reduces the weight even more.
Also the MP7 and MP5 are SMGs. That's what you call fully-automatic weapons chambered for a pistol calibre.
MP, as used by German companies to label German guns, stands for Maschinenpistole, which translates to submachine gun.
To anyone wondering why the new MP5 does so much more damage than the MP7:
9x19mm vs 4.6x30mm. The 9mm of the MP5 is approximately twice the size of the MP7's 4.6mm round, despite not shooting it nearly as fast.
I hope you find that helpful.