Unwieldy portfolio yield poor return

Unwieldy portfolio quality suffers as you own inferior investments along with good investments. This produces mediocre return or worse. 

i. Reduces Quality: There are not many quality companies and even lesser are available at cheap valuations. The more stocks you put into your portfolio, the less concentrated your portfolio will be in the best companies.

ii. Leads to Indexing: The more stocks you own the more correlated your portfolio will be to market returns. While passive management or indexing might work in bull markets, it does not work well in flat or bear markets.