Purpose: This study contributes to the existing literature by providing a comprehensive overview of the complex relationship between level of bank sizes, factors, M&A performance, and stability for banking sectors, Islamic vs conventional banks, and by highlighting the importance of considering the specific characteristics of the banking industry in GCC and Pakistan. Theoretical framework: The conceptual framework is developed and designed based on the theory and in line with the literature review. The theories are resource dependency theory and efficiency theory Design/Methodology/Approach: This paper employs an unbalanced panel data of 24 banks consisting of 10 Islamic banks and 14 conventional banks from GCC and Pakistan, 2004Q1 to 2020Q4. Data is collected from several secondary sources, namely Bloomberg, FitchConnect database, Bank’s financial statement, IMF, and World Bank database. Findings: Our results revealed that bank size has a significant impact on the M&A performance and stability of banks. However, there is no significant difference between pre- and post- M&A performance. Interestingly, smaller banks outperformed larger and medium-sized banks in terms of M&A performance, while larger and medium-sized banks exhibited better bank stability than smaller banks. Interestingly, while looking at Islamic vs conventional banks point of view results show that operational performance of Islamic is better than conventional banks. On the other hand, stability of conventional bank is better than Islamic banks. Research, Practical, & Social implications: The limitations of this research should also be acknowledged and future research should expand the number of observations and including more Islamic banks is essential. Moreover, exploring the impact of cultural and regulatory differences on M&A activities is also an interesting avenue for further research. Additionally, future research should investigate the impact of M&A activities on other performance measures, such as efficiency, productivity, and profitability. Originality/Value: This study aims to fill the theoretical and empirical research gap by examining the impact of level of bank size on pre- and post - M&A activities in the GCC and Pakistan for banking sectors, Islamic banks vs conventional banks.