According to iShares website, ITOT has $3,677,059,885 in assets. Having 3.7 billion in assets should result in very small bid-ask spreads, so I would trust the much higher number. I relied on Google Finance for years, but some of the details need fixing there (take a look at their P/E ratios, for example).
If anything, VTSAX and ITOT are almost the same thing (US total stock market, 3700 vs 3800 stock holdings). At Vanguard, check your commission cost for buying non-Vanguard ETFs (for example, it starts at $7 / trade).
https://personal.vanguard.com/us/whatweoffer/stocksbondscds/feescommissionsI would recommend you compare actual annual expenses.
For example, with $10,000 of VTSAX the annual 0.05% expense ratio works out to $5/year. If you move it to ITOT, you'd pay $3/year. But it probably costs you $7 to switch, so you pay $7 on the trade that takes you more than 3 years to break even.
But if you had $50,000 VTSAX, now you're looking at $25/year and the less expensive ITOT would cost $15/year. Under that situation, you save money within the first year after the switch.