Re the relative sizes of Sony, Canon and Nikon. All the figures below are in billions of Yen. (Currently, 1 billion Yen = £7 million.)
Total sales in the most recent full year published: Sony 7603, Canon 3401, Nikon 749.
Sony breakdown: mobile phones 759, gaming 1650, imaging 580, home entertainment 1039, semiconductors 773, components 195, movies 903, music 648, financial services 1087.
Canon breakdown: office 1808, imaging 1095, industrial and others 498
Nikon breakdown: precision equipment 248, imaging 383, instruments 73, medical 20, other 24
Sony's semiconductors division makes imaging sensors and processors for cameras, but also sensors and stuff for mobile phones, tablets etc. and other things. I can't readily find a breakdown. I think it's likely that Canon's sales of imaging products are larger than Sony's sales of imaging products plus related semiconductors, but probably not by much, maybe 20% or so. Both clearly dwarf Nikon.
In very round numbers, imaging products account for 1/2 of Nikon's sales, 1/3 of Canon's sales, and 1/5 of Sony's "technology" sales (i.e. excluding movies, music and financial services).
Sources:
Sony (year ending 31/03/17)-
https://www.sony.net/SonyInfo/IR/library/fr/16q4_sony.pdf
Canon (year ending 31/12/16) -
http://www.canon.com/ir/results/2016/rslt2016e.pdf
Nikon (year ending 31/03/17) -
http://www.nikon.com/about/ir/ir_library/result/pdf/2017/17_4qf_c_e.pdf