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Why liquidity implies tight spread and low slippage
You have just answered your question in the last sentence of your question:  More volume just means more people are interested in the stock...i.e supply and demand are matched well. If the stock is illiquid there is more chance of the spread and slippage being larger. Even if the spread is small to start with, once a trade has been transacted, if no new buyers and sellers enter the market near the last transacted price, then you could get a large spread occurring between the bid and ask prices. Here is an example, MDG has a 50 day moving average volume of only 1200 share traded per day (obviously it does not trade every day).  As you can see there is already an 86% spread from the bid price. If a new bid price is entered to match and take out the offer price at $0.039, then this spread would instantly increase to 614% from the bid price.