About the comparison between the two countries, I've found this site -->
http://country-facts.findthedata.com...Italy-vs-Spain
that summarizes a lot of different data. Anyway, I don't know how much accurate it is.
About this, I don't know how accurate this evaluation is for Spain, but for Italy I would not trust that number too much.
I'm quite sure that improb already named it, but I want to repeat it: don't underestimate the "lavoro in nero" in Italy (wikipedia calls it "
Unreported employment" )
In this case the problem is spread all over the country, and in many different ways. On the one hand there are people whose job condition is completely unreported, and that officially don't have any job nor earning. They can be workers hired with this modality, usually workers in the building sector of in agriculture, or domestic workers (people you hire for keeping your house clean, or for looking after old people for example). In this case there are also situations in which the workers are on the edge of work exploitation.
Other people whose work is completely unreported are some people who work in their own such as carpenters, plumbers, I also know some hairdressers that work like this. You recognize them because, when you pay them for their work, they never give you any receipt (no receipt = no taxes)
On the other hand, there are situation that are midway between tax evasion and unreported work. In practice there are a certain number of people that officially have a regular job, but that claim to have a much lower income than their real one for paying less taxes. One of the easiest example are are the bar owner that don't make the sale receipt for all the drinks he sell. This is a method for hiding a part of your income to the State, and for paying less taxes.
Another expedient is to make a sale receipt that report a lower payment than the real one.
This method is used by a certain percentage of people who work in their own, such as people who own a restaurant, bar, shop... , little entrepreneurs and freelance workers such as lawyers, architects, private doctors and dentists and so on.
For example a quite common situation when you have to pay a private doctor or a dentist, or a notary, they will ask you: do you want the receipt? If you answer yes, you will probably have to pay an additional price for it.
But how many of them are tax evaders? Right now I have only found
this article, which is dated 2012. So the data can be a little old.
Anyway, if you want some numbers:
Taxes were evaded by the 67.3% of gardeners, 62.8% of carpenters/woodworkers, 62% of plumbers, 60.1% of construction workers ... and the lower among the artisans are the installers of heating-cooling plants with the 18% of tax evaders.
Among the business owners, the 17.8% of bar owners didn't make the sale receipt (*this was in 2012, after that there was more attention toward bars), followed by the 17.2% of restaurant, pubs and pizzerie.
Among freelance workers, the 42.7% of lawyers didn't make the fiscal receipt, followed by the "geometri"(*) 40.2%, psychologists and psychiatrists 40%, architects 38.7%, dietitians 38%, specialist medics and dentists 34%. Then there are veterinarians 25.3%, "commercialisti" (**) 23.5% and notaries 19.6%.
(*) the sector of the "liberi professionisti", that in here I have translated as "freelance workers", is really complicated. Anyway, the "geometri" are technicians specialized in land use (for example you have to see a geometra if you want to make a new building). They are really good at technical drawing too.
(**) ironically, the "commercialisti" are the people you call for the papers related to your earnings. you can also ask them to write your papers for the taxes (tax statement and so) for example. Often tax evaders need the collaboration of a "commercialista" for evading.
In conclusion, in Italy there is a really large hidden economy and tax evasion. Some people do this because taxes in here are really high and so in some cases it is hard to survive. There are several cases of people who declared bankrupt because the taxes were too high.
On the other hand, there are also people who evade taxes and that are really rich. For example there are people that claim really low incomes, but that own huge cars and a second house in a famous place.
Anyway, the situation is really complicated and there are a lot of different situation, in general we can only say that tax evasion and unreported employment should not be underestimated in here.