Dabura667 is right on the money as usual.
However if you want to make a larger seed (like 256 bits) you *can* do that. The make_seed command line option will generate larger seeds:
electrum help make_seed
So you have 128 bits (+15 bits extra thats irrelevant) + key stretching ?
Am I correct?
What 15 extra bits? NVM
There is a discussion
here on this subject. Some of it refers to the old 1.9 branch. One thing that stands out:
Private keys only have 128 bit key strength. Not 160 bit and not 256 bit.
and this too:
In either case brute forcing the seed would allow you to gain access to all the private keys, where as brute forcing a single private key only gives you access to that key. It is generally speaking academic if the seed has sufficient entropy because keys with 128 bit key strength are considered beyond brute force anyways.