I really don't understand how anybody can believe this. Surely it must be a consequence of living in a big country and being used to comparing yourself with Germany and France.
In reality, Scotland has a very average size for an independent country in northern Europe. Have a look at the graph on the left, which shows the population sizes of various northern European contries (it's Scotland in blue).
Of course Scotland won't have the same influence as England, but similar countries such as Denmark, Norway and Ireland typically feel they have plenty of influence.
I definitely don't know of a popular movement in any country the size of Scotland that advocates joining a bigger neighbour because their country is too small to remain independent. Even very similar countries with a long shared history, such as Denmark and Norway, never seriously discuss becoming one country again.