Gassiev tipped the scales at 104.2 kilograms (229.7 pounds), while Kadiru weighed 116.7 kilograms (257.3 pounds), handing the late replacement challenger a significant size advantage heading into the main event at VTB Arena.
A 28-pound difference can affect clinch work, physical strength and ring positioning, but it can also reduce mobility and conditioning over twelve rounds.
Gassiev has often relied on compact combinations, timing and heavy hands rather than sheer size, making Saturday’s physical mismatch an intriguing tactical test.
Kadiru stepped into the title fight after Tony Yoka withdrew because of a back injury. Gassiev is making the first defence of the WBA heavyweight title he won by stopping Kubrat Pulev in six rounds at IBA PRO 13 last December.
The weigh-in attracted several high-profile guests, with eight-division world champion Manny Pacquiao joined by former heavyweight champions Shannon Briggs and Dereck Chisora.
All fighters successfully made weight before completing the traditional face-offs ahead of Saturday night’s card.
The heavyweight co-main event will see Joe Joyce meet unbeaten Russian Artem Suslenkov over 12 rounds for the WBA Continental heavyweight title. Joyce, the Rio 2016 Olympic silver medallist, is looking to strengthen his position in the heavyweight rankings against one of Russia’s rising contenders.
The card also features Sharabutdin Ataev against former world champion Jose Uzcategui in a 12-round WBA light heavyweight eliminator, while Aleksei Egorov faces David Dzukaev in a 10-round cruiserweight contest.
Elsewhere, Pavel Sosulin meets Eddie Colmenares for the IBA.PRO International super welterweight title, Muslim Gadzhimagomedov faces Kevin Martinez in a heavyweight bout, and Murad Khalidov takes on Arslan Yallyev in an all-Russian heavyweight contest.
Vadim Musaev boxes Farahat Manirola at welterweight, Tamerlan Ozdoev meets Shivam for the WBA Asia East lightweight title, and Vyacheslav Rogozin faces RV Deniega in a six-round bantamweight fight.
