Let's say stock XYZ has a bid price of $15.00 with a size of 400 and an ask price of $15.50 with a size of 1100. My understanding from reading on Investopedia is that these size numbers (400 and 1100) are the aggregates of pending limit orders~

These numbers usually are shown in brackets, and they represent the number of shares, in lots of 10 or 100, that are limit orders pending trade. These numbers are called the bid and ask sizes, and represent the aggregate number of pending trades at the given bid and ask price.

Moreover, it says~

The aggregation is for all bid orders being entered at that bid price, no matter if the bids are coming from one person bidding for 2,500 shares, or 2,500 people bidding for one share each. The same is true for the numbers following the ask price.

But I find this a bit confusing. The sizes are always some exact number of hundreds. Why is it always 400, 1700, 600, etc, and never 31, 845, 212, etc? Is the market maker in the middle rounding up and picking up responsibility for the excess?

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    From the same source: investopedia.com/terms/l/lot.asp Also note that if you wanted to hedge a position with a equity option, each option contract is for 100 shares. – Morrison Chang Jan 8 '18 at 0:53
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    I always place my orders based on what my position size spreadsheet produces, often being odd quantities like 847 or 286 etc. And I see these orders in the market depth after I have placed them along with other odd numbered orders. – Victor Jan 8 '18 at 1:13
  • "The sizes are always some exact number of hundreds." where did you see this? – Dheer Jan 9 '18 at 14:15

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