Regarding your concern about the full scan being as quick as doing a quick scan, yes this is normal due to the scan flow mechanism of the Trend Micro program. If there are no changes or modifications for the already scanned files, then those will be skipped when a second full scan was done right after the first full scan.
Other criteria for a file to be excluded would be that: (1) There are no new Trend Micro updates; and (2) the Trend Micro components have already performed its initial scan.
I hope this answers your question. Should you have any other concern, please do not hesitate to reach out to us.