Multiple sources (e.g. IEX Square Edge | Looking at the SEC's Proposed Market Reforms on Round Lots)
mentioned that NBBO quotes in SIP feed are round lot only but while I check for alpaca data A LOT of the quotes are sized less than 100. What’s the reason behind? One example is provided below. I think having this kind of clarity is really helpful to use TAQ data.
And similarly, it looks to me the depth (quantified by ask size and bid size at top of book) is much smaller than reality. I check using my trading app (containing ARCA book and Nasdaq ITCH), even at the NBBO the order volume is much higher than alpaca data feed (in above picture only 15 to 30 shares in ask side is way smaller than my trading app shows).
So a fundamental question is how alpaca computes ask/bid sizes?
Got some partial answer from here: Does Alpaca have NBBO quotes from *all* exchanges? - #8 by Dan_Whitnable_Alpaca
So short answer is quotes are from single exchange at any time point, not aggregated by price.
Still not sure why odd lot is observable in sip feed.
Hey 
The Alpaca quote sizes are actually in round lots: the first ask size of 15 in your example means 15 * 100 = 1500 shares.
I’m going to update the documentation to make this clearer.
See also Is the ask/bid size represented in round lots or as individual shares?.
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Thanks! Now I fully understand.