Every memory cell of a flash memory is having a capacitor which holds the stored information. Every capacitor has paristary resistor, so every capacitor is unloading itself. You can try yourself, take a capacitor, load it to it's maximum voltage and put it unconnected to anything aside on your desk. Look the voltage after some hours, after some days and compare the results. You see it unloads even if the both sodes of the capacitors are not connected to anything.
And now if you look at the flash cell circuit diagram, you may not see the capacity, but just some transistor. But as every transistor is also a capacitor (to be exact: there are three parasitary capacitors, they can be found between collector, ermitter and base on bipolar and less obvious to see at source, drain and gate of a field effect transistor), this is the so called "Miller effect", or "Miller capacity".
On normal transistor, used for switching or amplifying, the miller effect is a bad side effect as it influences the low signal behavior of the transistor what makes the transistor not behaving linear in frequency behavior, you may have heared of amplifier disortion. That is based on miller effect.
Your Hifi amplifier has THD (Total harmonic distortion) (German: Klirrfaktor) - these bad miller effects of the amplifying transistors is the reason for that. Transistor has no linear amplification for different frequencys because of Miller effect.
But for memory cell used transistor the miller effect is the thing which effeclty stores the information "0" or "1" you wat to keep. So you can imagine, the longer timer the flash cell should store the information, the bigger the miller capacity must be to sabe the "1". The bigger miller capacity, the bigger structures in the flash memory per surface, the lower the flash memory's capacity in bytes per surface. And the bigger the capacity is, the more time the capacity needs to load to get full (with the same loading resistor) when writing a "1" and to unload to write a "0". "Writing" low or high information takes longer the bigger the capacitor is. Speed of the flash memory decreases.
But on the other side, if you want faster access to the cell, the capacitor must be smaller to load/unload the voltage faster. The smaller you make the structures of the flash cell to increase the storage capacity per surface, the smaller the capacitor gets. The smaller the capacitor, because of increased speed and storage capacity (gigabyte) the smaller the miller capacity has to be, the faster it looses it's stored information because of parasitary resistance. This is very very logic.
https://en.wikipedia.org/wiki/Flash_memoryhttps://en.wikipedia.org/wiki/Miller_effecthttps://en.wikipedia.org/wiki/Parasitic_element_(electrical_networks)
https://en.wikipedia.org/wiki/Total_harmonic_distortionhttps://en.wikipedia.org/wiki/Small-signal_modelIf you don't believe, slip down my hump.
