The consensus I am seeing in Analyst Reports and Views is that 2020 is the year for Mid and Small Cap. The reason for the unifying stand seems to be based on "Mean Reversion"
Lets go back a bit in time. In 2011, markets went down - basically every Index went into negative territory. Nifty 50 was down 24%, Nifty Mid Cap 100 was down 30% and Nifty Small Cap by 33%.
Markets recovered pretty strongly in 2012 before starting to correct back in 2013. In August 2013, this was the returns of the 3 Indices (Absolute Percentage Move). 
The Best performer was not the worst performer of 2011. Rather, it was Nifty 50 which too had fallen quite a bit in 2011. The worst performer - Nifty Small Cap 100
While there is a issue in comparing small cap or even mid cap indices vs large cap on the long run due to good stocks getting out of these indices and bad stocks getting added, 20 months is small enough to ignore such biases.
If you observe history and not just of the Indian Markets, one pattern you find repeatedly is that Small Caps outperform everyone else in the last leg of a major bull market.
Small Cap stocks have various things that go against them. Some of them are lack of Institutional holding, low Corporate Governance Standards, generally lower promoter holdings among other
All these factors make it a risky proposition and hence tend to be ignored till the time that the bull run has been for a while and the differential in valuation is too big to ignore.
With new investors rushing in to have a piece of the pie, small cap stocks make eminent sense to invest into since they haven't yet risen as much and appear cheap.
But with low liquidity, big money can push up stocks literally on a 'J' curve. From being undervalued, these stocks basically to to overvalued in a very short duration of time.
When the crash finally happens, there aren't buyers to pick up the slack and stocks crash as if the company has gone bust.
90% of those who fall 70% or more and belong to the small cap category barely (if ever) recover. They were just hyped up stories with no real numbers to back it up.
10% are those that have value but thanks to the overall negativity they too get hit and while they shall recover in time, its a long time coming unless the market dramatically recovers.
Betting on Small Cap stocks deep research and understanding. Its not everyone's cup of tea. Remember the phrase, Markets may remain Irrational longer than you can remain sane.
For stocks that have fallen off the cliff to recover needs a trigger and unless you know the trigger (and it better be big), it pays to wait for the market to notice it before you start noticing.
A stock that has gone down 90% can go down another 90%. Mean Reversion isn't a guarantee, its just a opinion that somewhere the stock may bounce.
