Table Formatting problem in my new design

Just change the first column width which you assigned at begin{tcolorbox}[tab1,tabularx={l||Y||Y}] from X to l like that

begin{tcolorbox}[tab1,tabularx={l||Y||Y}]

You get the output like this

Keeping the three columns at X and Y widths will ensure that the three columns widths are equal, which you don't want. Assigning l for the first column width will let have its normal content width without breaking its cells lines, and then only equate widths of the 2nd and 3rd columns.

Maybe you'll like it:

documentclass[xcolor={dvipsnames, table}]{beamer}
usepackage[T1]{fontenc}
renewcommand{rmdefault}{ptm}
usefonttheme{structurebold}
definecolor{myblue}{RGB}{0,82,155}

mode<presentation> {
usetheme{CambridgeUS}
usecolortheme[named=myblue]{structure}
setbeamertemplate{navigation symbols}{} % To remove the navigation symbols from the bottom of all slides uncomment this line
}
setbeamerfont{table}{size=footnotesize}

usepackage{siunitx}
usepackage{booktabs,tabularx} % Allows the use of toprule, midrule and bottomrule in tables
usepackage{xparse}
NewExpandableDocumentCommandmcx{O{X}m}
    % O: optional, if it is not given, than is used default in {...}
    % m: mandatory
    {multicolumn{1}{>{centeringarraybackslashbfseries}#1}{#2}}
usepackage{tcolorbox}
tcbuselibrary{skins}

tcbset{tab1/.style={fonttitle=bfseriessmall,fontupper=footnotesizermfamily,
colback=green!10!white,colframe=red!75!black,colbacktitle=green!40!white,
coltitle=black,center title,freelance,frame code={
foreach n in {north east,north west,south east,south west}
{path [fill=green!75!black] (interior.n) circle (2mm); };},}}

tcbset{tab2/.style={enhanced,fonttitle=bfseries,fontupper=footnotesizermfamily,
colback=green!10!white,colframe=red!50!black,colbacktitle=Salmon!40!white,
coltitle=black,center title}}

begin{document}
begin{frame}[fragile]
sisetup{group-four-digits}
    begin{tcolorbox}[tab1,tabularx={l || S[table-format=6.0]|| S[table-format=6.3] }]
textbf{Description}    & mcx[X||]{1st Revised} & mcx{2nd Revision}    
    hlinehline
textbf{Construction Cost} 
                        & 598 762           & 620 991               
    hline
SI{10}{%} External Development Charges
                        & 54 449            &  56 398               
    hline
textbf{Sub Total}      & 653 211           & 677 388               
    hlinehline
Price variation of allotted works  
                        & {--}              & 6 502                 
    hline
Price variation of allotted works G-1 
                        & {--}              &   0.946               
    hline
External Electric connection payable to WAPDA 
                        &   5 000           &   5 000               
    hline
Add for Sui Gas connection charges 
                        &   3 000           &   3 000               
    hline
SI{2}{%} Consultancy fee for resident supervision
                        &  13 064           &  13 548               
    hline
SI{1}{%} for Tree Planation 
                        &   6 532           &   6 774               
    hline
SI{5}{%} PST          & {--}              &   8 654               
    hlinehline
textbf{Grand Total}    & 671 346           & 721 813               
    hlinehline
    end{tcolorbox}
end{frame}
end{document}