TBS001 12
TBS002 50
TBS003 20
TBS004 45

TBS001 10
TBS002 50
TBS003 20
TBS004 40

kent$  awk 'NR==FNR{a[$1]=$2;next}$1 in a && $2<a[$1]{
       printf "increase Tablespace %s by %d MBn",$1,(a[$1]-$2)}' f f2
increase Tablespace TBS001 by 2 MB
increase Tablespace TBS004 by 5 MB

$ awk '
NR==FNR{a[$1]=$2;next}
$2<a[$1]{print "Increase Tablespace",$0,"by",a[$1]-$2,"MB"}' source target
Increase Tablespace TBS001 10 by 2 MB
Increase Tablespace TBS004 40 by 5 MB