According to the analysis, it is the "performance-led culture" at Barclays that is to blame for this shortfall - with the bank more likely to enter into highly-geared, debt-heavy deals than its peers.
News of the potential future capital-raising follows the $9 billion rights issue completed by Barclays in July, which brought up its Tier 1 capital ratio to 6.3 per cent.
In the note, RBS analyst Ian Smillie commented: "[The] deeply ingrained performance-led culture facilitated the generation of [$16 billion] of economic profit over the last four years.
"It has, however, also led Barclays to a higher level of balance-sheet gearing than peers, an uncomfortable position in the current environment of financial system de-leveraging and heightened external scrutiny of banks' balance sheets."
Mr Smillie added that the bank might now launch a new rights issue in order to make up the shortfall.